Richard Riley wrote:
I'm looking for an engine that may not exist. If anyone has a notion
of what it might be, I'd be grateful.

It's for a 103 legal ultralight.

60 lbs or so all up, including re-drive and cooling
35-40 hp.
4 stroke.
More reliable than a 2 stroke Rotax

Any ideas?

I don't know if it's an exact match but the Briggs & Stratton V-twin is
close. They're very detuned in stock condition but the cart racers have
pumped them up considerably with little effort.

Tony