Cool project and a hell of a nice workshop!! Money doesn't appear to be a big issue here, so how about the Honda four strokes? Easier mounting, no problem with inverted running, and more power. One user told me when running his inverted, a very slight amount of oil drips down into the combustion chamber after you turn them off. Then when you start them they put out a very scale looking puff of smoke for a moment.

Good luck,
AV8TOR