Thanks for all of the info everyone! I'll probably swap the upgrade into my working 800XL if I can't get this one going.
ClausB - I think this one had a spill into the keyboard, and then they put it in a basement for 30 years. The shield was almost rusted off, and that rust is on a lot of the motherboard. The keyboard connector (on the mobo) has been used to the point of destruction (bent pins, etc) the mylar keyboard connector was rigged up with scotch tape, etc. It seems like if you took the time to install upgrades, it could have been kept free of liquids!
I've located the keyboard and connector, but $35 later it's still got a case that's in pretty bad shape - the cartridge port doors won't work because all of the support hardware is rusted off (springs, mounting screws, etc.), and I had to drill one of the screws out because it was rusted in.
I suppose I could swap the parts out, but after looking on ebay (I know, not the best source of affordable parts), the parts option ends up being expensive. I've tried bidding on a few of the 'good deals', but someone always steps up at the last minute to pay too much for some untested broken machine.
Anyway, I'm not going to part it out just yet, I can keep looking for a junker for a while and hope for the best!