I'm running a bit behind on this thread; here are thoughts about a
couple of questions.

Someone mentioned that the industrial engines are a bit heavy. True to a
degree, but if you're designing a plane around the engine (as God
intended) the extra weight is only a minor penalty. Early VW powered
designs had to make do with not much more than 40 hp, & the VW weighed
over 150 lbs. Hundreds of 'Piets' are flying with 35-40 hp Model A
engines that weigh way more than 300 lbs, IIRC. The plane was designed
for the engine. Hanging one on a plane designed for a Rotax won't be too
successful, though. :-)

Someone else mentioned using a heavier cam drive train as the prop
reduction drive. It's been done on certified engines. The problem I've
heard about is that it increases the likelihood of torsional resonance
(supposedly the reason that reduction ratios are almost always some
weird number).

Charlie