Welcome!
Sounds probable that it could be the key, usually takes a sudden stop to shear it, and is not overly common on these little ones, the flywheel is really light. What did you take apart?
Also worth checking the valve clearance, possibly points and condensor too.
Replacement key is 222698 genuine briggs and stratton, available from any mower shop or heaps of places online.
To remove the flywheel, you will need to remove the starter cowl, the undo the starter cup. now this either requires the special tool, or you can use large multigrips. You need to hold the flywheel to undo it but Don't use a screwdriver through the fins, they will break off.
Once the starter cup is off you will be able to see the keyway and whether it is sheared or not.
To fully remove the flywheel, it is on a taper on the shaft. so you will either need a genuine puller, or what I normally do is lever gently up on one side, and gently tap the other side with a hammer. what ever you do do not hit it hard, it will crack. If you hit the end of the crankshaft it will burr and the starter clutch won't go back on. All else fails there are videos on youtube on flywheel removal.
They were common once upon a time, getting less everyday, especially with good wheels. Doesn't mean its worth anything though, Rovers don't have the value of victas. I've only been collecting the 70's push mowers for the last year or so, they are out there but not like you would think.
