Check out the Honda four stroke personal watercraft engines. They're
powerful and designed to run at full load for extended lengths of time.
However, you'd have to use a reduction unit to get the RPM down low
enough to drive a prop.

Tony

tffy wrote:
This is for a hypothetical ultralight design, so just looking for
suggestions here of where to look at... hopefully a 4-stroke, purchase
price doesn't matter much, certification status doesn't matter either.
Long time before overhaul matters A LOT... so, the longer the better.
Is there anything out there in the 80-hp range? How about a motorcycle
engine conversion - how long can those go before needing to be
overhauled?

Thanks!