My Briggs and Stratton riding lawn mower engine is not starting.

Briggs and Stratton motor won’t start. It is a 14.5 hp 287707 engine. We have adjusted the valves, set the magnito, replaced the spark plug and cleaned the carb. The engine was turning over and backfiring so we pulled the flywheel and replaced the flywheel key which was sheered. Now it cranks and when it fires the engine pushes backwards (bounces) shredding the starter gear. Anyone have any ideas?