631 is the GE part number, but it cross references to a LOT of different part numbers. #89 is the usual part number that you'll see most often.
Essentially any BA15 style lamp with a 12v-14v rating will work.
Whomever worked on that CPU board in the past looks like they do good work. New sockets where needed, used a 6802 processor to eliminate the 6810 RAM chip, 3-chip ROM set, and most importantly your 5101 is in a socket too. Stupid easy to convert that game to an NVRAM, just remove the batteries and plug in the new chip.
-Hans