I know everyone is thinking this, so I'll just go ahead and throw it out there.
Go out and buy 24 chainsaws, all the same color. Take off the blades and hook them up to the rear wheel.

I think the engine cases for a 350 and 400 are the same? It might just mount right up, you may want to look one over.
If you don't mind cutting and re-locating your mounts, you can probably stuff whatever will physically fit in there...250 Ninja, Honda CB1, maybe a two stroke dirtbike engine? Sounds like fun.