Dan wrote:
The requirement stated by the original poster was more reliable than a
Rotax. Are we all that sure that a motorcycle engine or a overstressed
lawnmower engine will be even as reliable? Rotax's are pretty damn reliable
if maintained properly.

Actually with selective use of parts and limiting the revs, I am sure
you could modify a lawnmower engine to be reliable and more powerfull.
Not sure it would meet his criteria, and not sure it would be worth the
work. As for as motorcycles, that would be a much safer bet. Many
Honda Gold Wings are driven past 100,000 miles with so so maintenance.
The newer Harley Evo engines will also make it past 100,000 miles but I
don't think you would want that vibration. There are a few V twins
used on some Japanise bikes that are smooth, especially the crotch
rockets.