Lawn Mower w/ B/S motor won't turn over

Hoping someone can point me in the right direction. I have a mower with a briggs and Stratton engine. It has a steel flywheel. when I purchased the mower it had a plastic starter pinion gear. That gear quickly wore out. (the teeth ground down to nothing). I replaced the starter pinion gear with the exact same PN gear, but when trying to start the mower the starter pinion gear would shatter into a million pieces. I then tried a metal starter pinion gear. It seems to fit into the steel fly wheel just fine, but won't turn over the engine. I should note that the engine had difficulty turning over frequently with the plastic starter pinion gear too, where it would just make growly noises and not actually turn over the engine. I have replaced the starter motor, but that didn't help.

Any help you can provide would be greatly appreciated!

Snarfsmojo