I'd made sure the spark plug is a good working plug first Jeff as they can fail anytime , then you could
take the rocker cover off and make sure a push rod isn't bent ,check the rockers aren't damaged.
Sometimes when the flywheel key breaks the crankshaft needs to be cleaned with a file around the key way
slot otherwise the flywheel won't sit down properly on the tapered crank end thus causing the new key
to shear again ,you could just remove the flywheel nut to make sure the key is still lined up with the
flywheel and crank key way then put the nut back on.
When the key sheared the first time the engine could have backfired because of advanced timing
so something may have got damaged internally ,so remove the rocker cover and check rockers and
push rods.
Some motors use metal timing gears and some are plastic like ,difficult to tell until apart. If there is a key on
the cam gear I wouldn't have thought it would have broken from a sudden stop or backfire.
With these Chinese motors it comes down to how much time do you want to spend on them ,I've
thrown out good motors that just needed a lower crank seal as I can get a good mower at the tip for $10.
Cheers
Max.
