That's a single-plug Saito FA-325R5, so earlier version than the more recent dual-plug FA-325R5D. A 20x8 prop would be a good starting point. You may need onboard glow to keep all the cylinders lit. It should clean up just fine with some alcohol (or fuel); would be a good idea to liberally oil the cylinders (through the glow plug holes) and crankcase (throught the carb). I use a 50/50 blend of air tool oil and Rislone, but there are other after-run oils. It's not too difficult to pull the backplate and inspect the innards if some doesn't seem right, but I would avoid tearing into it.