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.

Great idea if you want to retire the reliable old Fox.  I lost a few of the old Fox's, mostly by impacting mother earth.  Still have a couple I think will still run with tender care.  How about pictures when done.

71 years of age.  Tell your Mom that Lew Woolard is still beating the socks off me in aerobatics at 85+ years of age.  Staying busy is what keeps us young.