One can not be timid when employing the "whack" method . To avoid doing any harm… preferably use a "knocker" screwed to the shaft end, if not available be sure that the flywheel nut is screwed on flush with the shaft end. Get a helper, have that person lift up on the flywheel, suspending the weight of the motor. The "whackie" must use a heavy hammer and give a solid blow squarely applied to the end of the shaft/ nut / knocker. A little heat applied to the hub of the flywheel will help, but shouldn’t really be needed.
Joe B