My L120 lawn tractor broke the ring gear that is under the flywheel the
starter engages with to start the engine. The one on the engine is held in
place by three rivets. The one in the parts book shows it is held in place
by four screws and nuts. The local dealer said to replace the whole
flywheel instead of just the gear.

Was this a parts change and no original parts are made for the engine?

The engine has this on it.

Model 406777
Type 0139E1
Code 040102YC