Forgive me for butting in, I have no experience or credibility on the
subject, but doesn't the HKS-700E fit right in this category? A
currently-in-production, honest-to-goodness, no-kidding, real airplane
engine; 60 HP, 100-odd pound 4-stoke twin cylinder, dual ignition,
integral gearbox with torsional shock apsorber and optional gear
ratios. I realize many will hack and wheeze at $10K price tag, but in
the world of airplane engines isn't this dirt cheap?

What are the fundamental problems with the HKS that it isn't one of
the options being discussed?

Gerry