Common situation on these engines.
Use lots of croil, oil filter strap and PATIENCE!!
I worked with mine for over two weeks. Sprayed on, put some force into the strap, gentle tapping with a plastic hammer on the dizzy to times every day.
Finaly it came loose. No rust, just old dirt, grease and oil binding it.
Try to no destroy the dizzy dataplate if it is present....
It will come loose...just dont use too much force.
Good luck
Svenn