"Richard Kaplan" wrote
So why dont' we see lots of homebuilts eliminating separate cylinders?

They do. Look at Rotax. It is essentially the standard for little
(Cub size and down) airplanes.

Also consider the Leeza Air-Cam. It is a professionally designed
airframe, purpose built as a camera platform twin for flight over
terrain that offered few viable forced landing options. Engine cost
wasn't really an issue. So what top-of-the-line certfied engines were
used? Right, none. They used Rotax.

Michael