Some Craftsman trimmers used a Briggs and Stratton 4 stroke. Is this one of them?

Others will have better ideas for keeping the prop driver from slipping. I guess I would drill and tap the hub for two setscrews that tighten against flats ground on the crankshaft. Position the setscrews 90 degrees apart. However, if you plan to convert to electronic CDI, then maybe the internal or external tooth lockwasher would work for trials. The hub/prop driver as drilled for the CDI sensor magnet will need to be positively retained against rotation to keep the ignition timing fixed. Good luck