I have never heard of airlines going bad. The ones that just came out of my jet when i "upgraded" to electric retracts and brakes were 20 years old and were still just as soft and functional as when they were new.

As far as the electrics, I would do a new RX and batteries. Depending on how much actual use is on the servos would determine if they get replaced. If they are still good and not worn I would keep them. But being 6V you will have to either use 6V batteries or add a voltage regulator of some kind. If you go new then everything can be high voltage.
With the age of the plane, look at all of the wiring If it is the old cheap #24 AWG wire I would upgrade it to all #20 AWG or better. Pull a few of the negative pins out of the connectors to inspect, if the copper is black, replace, if still copper or silver colored they are still good.