If it was mine I would use an air hammer without any locking device. That would rely on the inertia of the flywheel assembly to provide the resistance.
Sorry folks, I forgot to thank you for the helpful suggestions. Many thanks, I may get to heating it, after first removing petrol tank and carbs (engine is still in bike).My problem is not locking the drive, but actually shifting the very tight nut, and maybe the blowtorch will have to be the way.