I've seen failures on cpu board that cause it not to boot. Best to find a beat up unit to swap boards with. Korg wanted way more than the units go for in nice condition and they were on backorder last I checked the price/availability on *that* board... The buttons crack often and a careful use of a solder tip can melt plastic back together enough to then overlay goop adhesive quickly in select areas to add strenght and make these good as new in a pinch! I think Korg still sells them maybe too. The switches normally can be cleaned with contact cleaning and a lot of tapping on the switch. Keep checking until they get to a tenth ohm resistance when on.
I also just worked on an M1R that had some bad solder in the midi and a flagrant oxidized pair of contacts on a coil in the output filter of the 12V supply. Cleaned that up and it worked swell.
I have some remaining parts from a scrap out. Analog board, jack board, keys etc.Contact