Airplane design.

-----------much informative stuff snipped---------
(till homebuilders got to them!)

Very true! Plus the ones that found their way into and onto dune buggies,
small airboats, old BMW motorcycles, and heaven knows what else. The same
problem is true of Corvair engines, of which I've even seem one mounted in
the bed of a pick-up truck and driving an electric generator. The BMW 600
and 700 automobiles were never popular enough to have single seat aircraft
designed around them, at least in the US, and the little air cooled Subaru
engines were probably gone from the stock-piles before the Part 103
ultralight rule allowed 254 pounds empty. I suspect that the BMW and Soob
engines would have been chosen over the 1/2VW if availability permitted.