I had this problem with a featherweight. I stood it on end with the flywheel up and filled the area with PB blaster. I let it sit for a week. Tried to wiggle the stop motion wheel. Let it sit some more. Tapped with a rubbger mallet. let it sit some more. Found an adustable wrench that would fit and voila! off it came.
Patience and a lot of oil/lubricant.