The old one will just slide off with a little energetic massage with a hammer & punch. Be sure the new one is heated enough to be a free slip fit as it's installed. If there's a shoulder on the flywheel, the new gear should drop onto it easily, and align with the shoulder. If not, be sure the flywheel is on a flat surface, so the gear will shrink itself into place in proper alignment. A wobbly ring gear due to misalignment during installation is a PITA to straighten out. I use a rosebud tip on an acetyline torch to heat the gear, but it can also be heated on a barbecue grill. Just don't heat enough to turn it red hot, as this can anneal the heat-treat of the metal and make it soft enough to wear quickly.
Jerry
