Hi Michael,
Before attempting any welded repairs (which could weaken the end of the crankshaft), I would set the crankshaft up in the lathe and clean up the taper. Then do the same with the flywheel (to remove the brass from the previous repair). Then turn up a tapered sleeve, and lap it to the crankshaft and the bore of the flywheel (some trial and error will be needed to get the correct clearance for the clutch operation). When you are happy that everything is fitting together correctly, cut a slot in the sleeve for the flywheel key. When fitting the key, make sure it is not too full, as that will hold the flywheel off the taper.
Good luck with the repairs,
Regards,
Eddie.