Matt try posting this under avionics--you might get some hits.  Wish I cud help ya but don't have a clue, except to say, if it costs more than about $50 its a real waste of money, with the prices of new solid state radios from King, Michele, etc.  you can get a nice new 720 for around $1000--maybe less for a used.  Good luck but don't pour money into a sinking ship!

Take a look at this site.  It has some pretty good info.
<http://www.avionix.com/navcoms.html>
In the past I have had very good responses to my emails and phone calls.
The TKM MX-300 is probably your best bet, as it should just slide in your existing tray and will (should) operate your VOR indicator.
Any other radio will need it's own mounting tray and have to be installed and wired.  That can get kinda pricey for an avionics shop to do the installation, sometimes more than the cost of the radio.

There's a Cessna 300 on eBay for one dollar
and a few KX 170Bs..with & without trays..some with indicators...

Good luck on your quest  :-)
Michael