If you've been keeping up with my low aspect ratio UL thread you'll see why I need more power than most ultralights, besides it might not even be ultralight

Anyway, a two stroke engine with dual plugs sounds like a possibility, I want around 40hp so that means a bigger engine or two normal ones, but two engines would need to be linked together somehow.

Concerning the four stroke adaptations of motorcycle engines, why do you say that they will need a lot of machining? To cut off the transmission? I have the worlds biggest angle grinder that could cut a car in half in 60 seconds (exaggerating)

For my needs it seems one of the 500cc water cooled singles off a dirt bike would be powerful enough, but what kind of weight are we looking at for something like that?