Finally cracked this fella open last week after a long wait to investigate some issues. The missing switch cap I already replaced was rattling around inside! So, spare switchcap.
There were a lot of screw problems. The back, the frame, the preamp, and the deck all had either stripped heads, stripped holes, in the wrong locations, missing insulating washers, not originally from the unit, or just straight up missing. So I spent the first day sifting through my screw collection and finding correct replacements for all of them based on service manual data.
I had expected the belts to need replacement since the deck was dead, but they were new. Just too big. One fell off and was why the deck wasn't running. Swapped them out with correct sizes and everything works fine. The idler tire and pinch roller were a little crusty so I sanded them down a bit and are nice and rubbery again.
On plastic stuff, a cabinet screw post had to be rewelded, and I replaced a missing dial string pulley. Very strange as they put back the little axle for it. At any rate, it's not essential to function, but helps keep the string straight as it comes off the tuner shaft. I found one with the same diameter but had to grind down one side to make it identical, and bored out the hole a bit to fit the axle.
I noticed a pair of 33-ohm resistors along the bottom edge of the preamp that didn't look factory, and sure enough they're not in the service manual nor on pics of other 920 boxes. So I removed them. No difference so far that I can tell. They were pulling the base of a pair of transistors on each channel to ground, but didn't seem to make any sense what they were trying to achieve.
Someone (maybe Sanyo?) also messed with a few of the air core inductors on the tuner board. FM tunes in very strong, but I'm not sure about the other bands, it may be fine.
Sanyo soldered the motor wires, so I added a connector to them to allow the deck to come out.
Sanyo also used a couple of lousy connectors for chassis-to-frame connections that you slip a tinned end of a wire into, but is nearly impossible to come back out without the use of a pick, so I may replace those with better ones.
Next up: figuring out why the deck only records the left channel.