Cut off the back of that crankshaft, and then the engine can mount flat against the firewall, using studs and washers/nuts behind the firewall as you mention. The three will hold it just fine, but use nyloc nuts, or Loctite on them. If you don't want to disassemble the engine, you can cut the crank off with a dremel and a cutoff wheel. Just go a little at a time so you don't heat the crankshaft up enough to damage that rear seal.

The three bolt crankcase was used on 16cc and early 21cc engines.

AV8TOR