Ring Gear
Steve ;
First thing , go find a commercial brake shop , the kind of place that re-lines brake shoes, rebuilds clutches and grinds flywheels etc. ~ they'll be able to press the old one off and the new one back on but best of all , they'll prolly have your new ring gear , in stock .
If i turns out to be a really oddball , unobtanium one , they can mark it , press it off , rotate it 90° in the direction the engine turns and re-install it , this will work for years to come as the engine always comes to rest in the same position (hence why it wears in one place) so rotating the ring gear 90° allows the starter's Bendix to engauge just after the damaged bit and by the time the damaged bit comes 'round , the Bendix is full meshed so there's no problem .
This is not some therory , it's a dealer trick I learned 40 years ago and have been using ever since , never failed .