Hi Steve. Fox mfg made several versions of that engine. Most likely you have the 36X that has one needle bearing at the rear of the shaft. Those engines ran very well with props in the 8 X 8 to 9X7 range and with nitro from 10% to 40%. oil should be 22% to 25 % and a mix of Castor and synthetic around 50/50. The hole with the screw in it in the back plate is for a crank case pressure titting. (most had abandoned the case pressure fuel systems by the time that engine came out, and were using bladder pressure systems (either Passifier, pippette tips, or surgical tubing). Fox mfg no longet has many repair parts for those engines. I do know that they still have the rear bearing, and the 36 sport piston/sleeve will fit but the timing is a little milder so if you are adept with a dremel tool and a file, you can still et the performance back with the sport P&C.
I have several of those engines and some spare parts in my junk barrel.
Bigiron
PS if you remove the shaft, be VERY CAUTIOUS to not lose the little rollers from the rear needle bearing. If they are worn even a wee bit, they may fall out.