So Subaru engines are no good because: they only produce the required power at high rpm (5000) and need to use a reduction gear (psru).
Well isn't that the case for the Rotax (aka Austrian hand grenade).
At least the Subaru will only cost 25 % of a Rotax unit.
What's is it in ? or going in ?
Alternatives: small Lycoming or Cont., Wam 120, Limbach etc...