I forgot to reinstall the lock mechanism for the MPU.

From an advanced technical perspective it is my experience that computer chips work on smoke.
When the smoke leaves the chip they no longer work and it’s extremely difficult to put it back in.

Yep watch what direction the chips are inserted.

It would be helpful if the directions were explaining the smokey thingamajig.

Because an LED is a solid state device and not a incandesent bulb.  With older electronics, LED's can cause goofy issues like you are experiencing.

you can buy a hopper now and wire it in ... worst case is the payout disc doesn't have the exact trace lengths you need to match the glass so your pays would be incorrect.  If you find a payout disc later, you'd just swap it in (soldering required).

the typical pay for a cherry on most machines is 2 instead of 3, and there's a lot of payout discs that support the other pays (5, 10, 14, 18, 150, 200).  The 150 is the next least common.

based on part numbers, there's at least 450 different payout discs.

the M-645-102 on the diagram in post #19 would work without modification, so would M-645-103. 

the more common M-645-96 below would only pay 2 instead of 3, as would M-645-141.

some versions of M-645-141 used on some model 831 machines may have room to lengthen the 2 trace with copper foil or conductive epoxy.  See second image below.  Some of the higher part number plates may work as they can have dual wiper arms and they were trying to make the plates more generic.

if you are asking parts suppliers tho, it's easier to just tell them what pays you need the disc to support and they can look through their box of discs.  If they have a disc that needs two wiper arms and you only have one, screwing a second arm onto the spiral cam is easy enough unless you have a really old game without the support for it, then you just buy a spiral cam with two arms.

The sockets on the board are marked showing which end has the notch.