Before installing the new gear, remove the old one. If it is only worn on one side, it may be replaced backwards and serve a long life.
To remove it, heat it with an acetylene torch, you have to heat it without too much heat reaching the body of the flywheel. that means a hot flame, moved quickly all over the ring gear. putting it on is tricky, but it can be done the same way. It will heat quicker to put it on, because you won't be heating the body as much.