> but clean the MP, inside and out, with alcohol every few months.
Don't do this. The alcohol will remove useful lubricants on
potentiometers and switches; those will become scratchy - even the
effect you are having now might be increased by having potentiometer
without lubricant(*). Also I would not want to test, how too much of
alcohol interacts with older capacitors.
Keep the MP dust free, and avoid any liquid on the synth in any case
(the video shows spots of tea or coke on the keyboard - these are
killers!). That is all.
> I suspect a destination switch might be difficult to replace.
I don't think you have to replace it. It should be easy to clean. First
remove grabable dust with a pair of tweezers; then use compressed air to
blow out the rest of dust (compressed air is even available as spray in
a can). Then apply Caig Deoxit as deoxidant and sealer.
Same with potentiometers, but use Caig FaderLube FL5 for cleaning and
Federlube FL100 for sealing.