Here's a list of STC'd engine conversions for the 172A from the FAA website. By far the most popular conversion is to the Lycoming O-360. Air Plains has an O-360 STC, but unfortunately it doesn't cover the 172A (the earliest 172 covered is the 172B). I'm not sure how many of these are currently active, but the Del-Air is, and I've heard very good things about their conversion.

Hope this helps.

STC Number: SA00461SE

This certificate issued to: Del-Air (Harry R. Dellicker)
Phone: (559) 784-9440

STC Holder's Address:
2121 S. Wildcat Way
Porterville CA 93257
United States

Description of the Type Design Change:
Install a Lycoming O-360 series engine and Hartzell, McCauley, or Sensenich propeller (See STC for specifications).

Application Date:

Status: Amended, 10/24/2002

Responsible Office:
ANM-100L Los Angeles Aircraft Certification Office Tel: (562) 627-5200

TC Number -- Make -- Model:
3A12 -- Cessna Aircraft Company, The -- 172G
3A12 -- Cessna Aircraft Company, The -- 172H (USAF T-41A)
3A12 -- Cessna Aircraft Company, The -- 172E
3A12 -- Cessna Aircraft Company, The -- 172D
3A12 -- Cessna Aircraft Company, The -- 172C
3A12 -- Cessna Aircraft Company, The -- 172K
3A12 -- Cessna Aircraft Company, The -- 172B
3A12 -- Cessna Aircraft Company, The -- 172L
3A12 -- Cessna Aircraft Company, The -- 172
3A12 -- Cessna Aircraft Company, The -- 172A
3A12 -- Cessna Aircraft Company, The -- 172F (USAF T-41A)
3A12 -- Cessna Aircraft Company, The -- 172M
3A12 -- Cessna Aircraft Company, The -- 172I
3A12 -- Cessna Aircraft Company, The -- 172N
3A12 -- Cessna Aircraft Company, The -- 172P
3A17 -- Cessna Aircraft Company, The -- P172D
3A17 -- Cessna Aircraft Company, The -- 175
3A17 -- Cessna Aircraft Company, The -- 175A
3A17 -- Cessna Aircraft Company, The -- 175B
3A17 -- Cessna Aircraft Company, The -- 175C
A-799 -- Cessna Aircraft Company, The -- 170A
A-799 -- Cessna Aircraft Company, The -- 170B

*********************************************

STC Number: SA293NW

This certificate issued to: Franklin Aircraft Engines, Inc.

STC Holder's Address:
136 Racquette Drive
Fort Collins CO 80524
United States

Description of the Type Design Change:
Installation of Franklin 6A-350-C1 or 6A-350-C2 engines, Hartzell HC-C2YF-1BF/F8459-4 to -8, HC-C2YF-1BF/F8459A-4Q to -8Q, or McCauley 2A31C21/84S-4 to -8 propellers.

Application Date:

Status: Amended, 11/09/1999

Responsible Office:
ANM-100D Denver Aircraft Certification Office Tel: (303) 342-1080

TC Number -- Make -- Model:
3A12 -- Cessna Aircraft Company, The -- 172A
3A12 -- Cessna Aircraft Company, The -- 172I
3A12 -- Cessna Aircraft Company, The -- 172P
3A12 -- Cessna Aircraft Company, The -- 172N
3A12 -- Cessna Aircraft Company, The -- 172M
3A12 -- Cessna Aircraft Company, The -- 172K
3A12 -- Cessna Aircraft Company, The -- 172
3A12 -- Cessna Aircraft Company, The -- 172H (USAF T-41A)
3A12 -- Cessna Aircraft Company, The -- 172G
3A12 -- Cessna Aircraft Company, The -- 172F (USAF T-41A)
3A12 -- Cessna Aircraft Company, The -- 172E
3A12 -- Cessna Aircraft Company, The -- 172D
3A12 -- Cessna Aircraft Company, The -- 172C
3A12 -- Cessna Aircraft Company, The -- 172B
3A12 -- Cessna Aircraft Company, The -- 172L
3A17 -- Cessna Aircraft Company, The -- 175C
3A17 -- Cessna Aircraft Company, The -- 175B
3A17 -- Cessna Aircraft Company, The -- 175A
3A17 -- Cessna Aircraft Company, The -- 175
A-799 -- Cessna Aircraft Company, The -- 170A
A-799 -- Cessna Aircraft Company, The -- 170B

*************************************************

STC Number: SA3-571

This certificate issued to: KWAD Company

STC Holder's Address:
4530 Jettridge Dr. N.W.
Atlanta GA 30327
United States

Description of the Type Design Change:
Lycoming O-360-A1D engine, McCauley 2D36C14/78KM-4 or Hartzell HC-C2YK-1A/7666-2 propellers.

Application Date:

Status: Reissued, 03/09/1979

Responsible Office:
ACE-115A Atlanta Aircraft Certification Office Tel: (770) 703-6035

TC Number -- Make -- Model:
3A12 -- Cessna Aircraft Company, The -- 172B
3A12 -- Cessna Aircraft Company, The -- 172H (USAF T-41A)
3A12 -- Cessna Aircraft Company, The -- 172C
3A12 -- Cessna Aircraft Company, The -- 172D
3A12 -- Cessna Aircraft Company, The -- 172E
3A12 -- Cessna Aircraft Company, The -- 172F (USAF T-41A)
3A12 -- Cessna Aircraft Company, The -- 172A
3A12 -- Cessna Aircraft Company, The -- 172G
3A12 -- Cessna Aircraft Company, The -- 172

**************************************************

STC Number: SA420CE

This certificate issued to: Williams Robert or Barbara

STC Holder's Address:
213 N. Clark Phone: 620-782-3317
Udall KS 67146
United States

Description of the Type Design Change:
Installation of O-360-() series engines, fixed pitch and constant speed propellers. (172D, 172E, 172F (USAFT-41A), 172G, 172H (USAFT-41A FLOATPLANES (EDO89-2000 FLOATS ONLY) (172A, 172B, 172C LANDPLANES)

Application Date:

Status: Amended, 10/31/1986

Responsible Office:
ACE-115W Wichita Aircraft Certification Office Tel: (316) 946-4100

TC Number -- Make -- Model:
3A12 -- Cessna Aircraft Company, The -- 172D
3A12 -- Cessna Aircraft Company, The -- 172
3A12 -- Cessna Aircraft Company, The -- 172G
3A12 -- Cessna Aircraft Company, The -- 172B
3A12 -- Cessna Aircraft Company, The -- 172C
3A12 -- Cessna Aircraft Company, The -- 172A
3A12 -- Cessna Aircraft Company, The -- 172E
3A12 -- Cessna Aircraft Company, The -- 172F (USAF T-41A)
3A12 -- Cessna Aircraft Company, The -- 172H (USAF T-41A)

****************************************************

STC Number: SA792WE

This certificate issued to: Johnson Joseph

STC Holder's Address:
P.O. Box 851
Kenai AK 99611
United States

Description of the Type Design Change:
Lycoming O-360 Series engines and Sensenich M76EMM propellers in Cessna 172 and 172A Model aircraft.

Application Date:

Status: Amended, 09/25/1997

Responsible Office:
ACE-115N Anchorage Aircraft Certification Office Tel: (907) 271-2668

TC Number -- Make -- Model:
3A12 -- Cessna Aircraft Company, The -- 172
3A12 -- Cessna Aircraft Company, The -- 172A

****************************************

STC Number: SA807CE

This certificate issued to: Williams Robert or Barbara

STC Holder's Address:
213 N. Clark Phone: 620-782-3317
Udall KS 67146
United States

Description of the Type Design Change:
Installation of Lycoming 180 hp engine and constant speed propeller. (172P LANDPLANES)

Application Date:

Status: Amended, 05/03/1990

Responsible Office:
ACE-115W Wichita Aircraft Certification Office Tel: (316) 946-4100

TC Number -- Make -- Model:
3A12 -- Cessna Aircraft Company, The -- 172P
3A12 -- Cessna Aircraft Company, The -- 172C
3A12 -- Cessna Aircraft Company, The -- 172D
3A12 -- Cessna Aircraft Company, The -- 172G
3A12 -- Cessna Aircraft Company, The -- 172N
3A12 -- Cessna Aircraft Company, The -- 172M
3A12 -- Cessna Aircraft Company, The -- 172L
3A12 -- Cessna Aircraft Company, The -- 172B
3A12 -- Cessna Aircraft Company, The -- 172K
3A12 -- Cessna Aircraft Company, The -- 172F (USAF T-41A)
3A12 -- Cessna Aircraft Company, The -- 172I
3A12 -- Cessna Aircraft Company, The -- 172E
3A12 -- Cessna Aircraft Company, The -- 172
3A12 -- Cessna Aircraft Company, The -- 172H (USAF T-41A)
3A12 -- Cessna Aircraft Company, The -- 172A

iM IN EAST AFRICA I WOULD LIKE TO CONVERT MY 172E CURRENTLY POWERED BY O300C TO LYCOMING O360.I DONT KNOW WHERE TO START AND ROUGHLY THE BUDGET FOR THAT
MAZRUI
PHONE +255 777 414366

Try David Stoots in Fairbanks Alaska (907)271-2668, he has an I0-360 comversion for the 172 and 172A

DEAR SIRS
COULD YOU PROVIDE US WITH THE INORMATION
TO CONVERT THE CESSNA SERIAL 172I57128 WITH 160 HP O320 ENGINE
TO 180 HP ENGINE UPGRDE
BEST REGARDS
MR VANKALLEN