The strength or size of the key is not what keeps the flywheel from spinning on the shaft.  The correct torque on the nut wedging the two tapers together is what keeps the flywheel from slipping.  The key's only purpose is to properly index the flywheel with the crankshaft so that the ignition timing is correct. 

The tapers suffer some damage when a flywheel spins on the crank.  Lap the tapers together with some valve lapping compound to make the tapers fit each other properly.  Proper torque on the nut will not prevent the flywheel from slipping if there is minor damage to the tapers.