I think I'd use something like a toothbrush handle. I wouldn't want to mar up the crank or crank case with a screwdriver jammed in there. You'd have to remove the carb, but would be less likely to damage stuff.
I have had that. Back before I understood that you should put shims behind the collet to space things out properly, I just held the flywheel where I wanted it, then did a quick jerk to get the nut to bite down the collet. Then the flywheel broke loose and my only option was to lock the piston...