I just upgraded the circuit boards and thermostats (digital) on 2 of mine. I found a guy on ebay that had 2 for $350. The circuit board is also available at Northwest RV Supply
http://www.nwrvsupply.com/Merchant2/...egory_Code=ele
for $119 but doesn't include the thermostat. My local a/c guy serviced mine last year and is coming out to clean/service them next week. He said that he should be able to get parts if needed. He also said that he could build a system to replace one if he couldn't find the parts. I plan to nurse mine along for as long as I can. Then I'll replace them with roof-tops. Steve Magown did his and ran the wire outside along the roof. I was thinking maybe run the wire (especially for thermostat) in wiring races along the metal trim strips to a nearby cabinet. You can get the races in a color that would match pretty close to the headliner.
Another thought would be one of the mini-split units that are becoming popular. You might be able to adapt one by destroying the cabinet and mounting it in place of the basement units. You could utilize your existing wiring, tubing and ductwork. Neither option would be cheap, but would retain the features of the basement unit.