Veeduber schreef:
Lots of interesting ideas coming out.

Indeed, certainly to the eyes of this non-mechanic.
But few seem to answer your original search for an alternative for the
VW: a light engine of 40-60 HP, typical for a single seater.

Or what did I miss?
For that application, the Citroen 2-cylinder aircooled boxer seems one
of the rare alternatives; though it is smaller than the VW.