.
Nobody wants to do it, but a weekend spent with several brass tooth brushes and a tube of Anti-Oxidation paste will solve a lot of electrical gremlins. start at the batteries. Clean them up, apply AntiOx and then disconnect everything wire by wire (one at the time), clean them, apply Anti-Ox, then reconnect.
It isn't sexy...
It isn't as cool as new big tires...
Nowhere near as nice as doors if it is cold or raining...
Nothing like a radio and big speakers so you can "jam" down the road...
But cleaning up the connections on a machine left out in the weather for 20 years will make it and you a lot happier.