Fred the Red Shirt schreef:

I am surprised that motorcycle engines, like the BMW footwarmers,
are not a popular starting point for conversions. I'm sure there are
good reasons for that, just don't know what they are.

Whatever the reasons, they must depend on local factors, such as
availability of engines, spares, workmanship. Here in tiny Belgium I
hear about spreading use of a BMW bike engine conversion.

http://www.asa-be.com/images/PhotoGa...G01/OO-G23.JPG

shows an example - observe the extra cowling width, and the very long
exhaust pipe.