I've dinked around with several engines in the garage and have always wished I didn't have to mount a transmission to spin the motor. I thought of a possibility and had some scrap steel and time so worked up a quick mount, that actually works. I had started out using a spare starter and the back of an engine for measurements then realized that the backing plate on the engine has all of the necessary holes, a ready-made template.
If you have a drill press, and a jig saw, it's not too difficult. A battery, starter, and the plate and I can spin a free-standing engine as long has it has a flywheel.
The steel is 3/16" x 2 1/2" mild steel, hot-rolled I believe. I showed the starter bolted to the plate but in use the plate has to go on first and the starter through the backing plate hole. I also bolted the plate to the engine backing plate when I drilled the engine mounting holes. Less error using the engine plate as a guide. Just another thought I wanted to get out there. Might be handy for someone.
Forgot to say, 3/16" is the limit on thickness. There's a sliver of space between the plate and the flywheel. No running, but no room either.