I don't remember what I did as a kid when I had the Whizzer Pacemaker, but I am wondering what is the best way to hold the flywheel from turning while I tighten the bolt? Thanks for any advice.
Use a 26" junk v-belt to hold it. In your minds eye imagine the following orientation. Place the belt over the motor pulley as it would normally be installed with it running back toward the clutch. Now pinch the belt together just to the rear of the flywheel pulley and wrap the tail of the belt below the pulley so that it is doubled over on itself. Stick a hammer handle or similar bar through the loop you've created in the belt, and as you rotate the flywheel counterclockwise it will tighten and capture the flywheel. As you tighten the bolt the handle you've inserted will be pulled up against the edge of the flywheel or engine case and come to a rest there. Reverse the orientation of the belt to loosen the flywheel bolt. I've never come across a flywheel I couldn't hold onto using the technique - poor mans strap wrench!
Don't forget there is a difference between the new edition and the vintage motor concerning the flywheel bolt. Vintage motors use left handed threads, whereas the new motors are right handed threads.