Bigger Briggs engines rarely break flywheel keys. That more of an issue with small engines direct coupled to a blade.
I'd suspect first - early igntion timing - maybe due to a bad Magnetron module - and/or - weak spark.
Break the electrode off an old spark plug, hook it to a plug wire, and see if a blue spark jumps the new enlarged gap. If it does not - it's in sufficient.
Stuck valves can give similar symptoms - but they usually show when an engine has run a while and gets real hot - then the exhausts can hang up. If the valves were rusted from sitting - it would not kick back like you describe.
In regard to valve-tappet clearance - just make sure you have at least enough to close and make sure you don't get a false reading due to the compression release built into the camshaft. Most aluminum engines are .007" intake and .011 exhaust. Most cast-iron engines are .009" intake and .014" - .017" exhaust.
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Today's Featured Article - Recollections of a John Deere GP - by Charlie Cook. I grew up on my dad's 280 acre general farm in Western Michigan. Dad's only tractor when I was very young was the old GP. My first recollection of this tractor was when dad allowed my brother and myself to ride on the tractor with him driving. One of us would sit on the left fender (away from the clutch) and other would sit on the gas tank and hold onto the crankcase breather pipe.
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