Its a 4 cylinder flat head engine? I did some research and the Continental F135 looks to be a lower displacement version of the ever popular F163 engine. Chances are its a press on ring gear. As an engine begins to get tired, the flywheel tends to stop in the same spot everytime. If you spun the flywheel 360 degrees with the starter off you will probbly find the rest of the flywheel looks ok. If you are unable to find the correct parts, you might consider pulling the flywheel, marking the current location of the worn area, knock the ring gear off the flywheel and reinstall it in a fresh area where it tends to stop. Much better would be to locate a new part. Good luck-