G'day BSA500.
Been a long time since I had this problem (25 years). I think I undid the three nuts holding the mag and used timber wedges between the mag flange and c/case. Then applied heat to the center of the advance unit as BJ suggests, screw the nut on a few threads and tap-----Hit-----BELT the nut.
I don't condone brute force but what has to be done, has to be done.
Cheers