Like the others point out, the hard part is splitting the tractor. After removing the flywheel replacing the ring is a snap:
1 - Heat the old ring gear lightly - a propane torch will be sufficient. The warmed gear will slip easily off the flywheel with a few light taps.
2 - Heat the new ring gear 15 minutes in a 400 deg oven. (Tip: this best done while your wife is off somewhere.)
3 - Now set the hot ring gear in place on the flywheel. It will grab tightly as it cools.