B&S 17.5 horse The Nylon gear is toast how to change it to a new one seems to be riveted to the cast iron fly wheel.I have it Off the motor so start from removel
From the fly wheel.

Hi,
There were lots of problems with the plastic composite ring gear(usually associated with hitting the starter with the engine already running.) There is a ring gear kit, part #399676 that includes a new pinion for the starter that must be used.
With the flywheel upside down on a work surface, center punch the heads of the rivets and drill off the head with a 5/16" drill bit. When drilled through, the remainder of the head will break free telling you that you are done with that one. Once all are drilled, remove the remainder of the ring gear, line up the new aluminum gear with the countersinks upwards and align the holes to the ones in the flywheel. Install the screws through ring gear and nuts to the top side(bottom with flwheel upside down) and tighten securely.
Put everything back together.
Peace,
Clarke