See if you can find a coupling nut that thread size. They are used to join lengths of threaded rod and are about as long as four standard nuts. If the hole in the flywheel is small’ you may have to trim the diameter at the bottom. Easy if you have a lathe. When it is screwed in place, thread a bolt in against the crankshaft. Then cut it off flush and stake it in place. Paint it a bright color so you won’t lose it.
