Screws are #3-56 1/4" stainless allen screws. Easy to find on Ebay. If you're ordering another part like a bare baseplate and an AC motor (don't forget base plate screws) then they will toss them in. I have a good supply. The original Bristol screws are a rarity, and I have a small supply.
I suspect you don't know the serial number of the scope, because it's recorded only on the base plate. The optics have code numbers, but that requires disassembly, and then the factory can look up the number.
A new base plate is about $25 from the factory, and an AC motor might be pricier new, or they may sell you a take-out from a PG conversion.
I'm a fan of the PG-1 you have because it's the only Questar to track without a cord dangling from the base. Even without the controller it still tracks as well as the AC motor. I have three of these and vastly prefer them.
If you are unable to acquire a PG-1 base plate with its special cutouts and milled pocket, I'd be interested in the PG-1 parts for my repair bin. Maybe I can help you get fully set up and assembled with AC if that makes sense.