Try talking to Mid West Super Cub (
https://mwsc.co/ ). They have an adapter that should fix your problem.
link to there catalog:
https://cdn.shopify.com/s/files/1/18...72684240155565
Go to page 14-right side-item 51168 SW.
The Magnum crank has a bolt on the crank end to hold the hub on. I have repaired a few cranks that broke the threaded end off by drilling a hole, threading it and used a bolt.
My guess is either you have a magnum crank or the threaded end was broken off.