I have to agree with this. Water cooled industrial or auto engines
are becoming the best option.

Our local building community has modified Rover aluminum V8's (Buick
215 for us old guys) and a number of Japanese engines. The success
has been ok but these are expensive conversions with trick quality FI
and Ignition systems supplied often by SDS. They are local as well
and easily accessable to the homebuilders here. Thier website is an
excellent source of conversion info, again high end but cooling and
conversion issues are well researched with lots of tech articles.

http://sdsefi.com/ the aircraft page link is at the bottom.

I might differ with Bob a bit as I am leaning to affordable PSRU
engines so I can get a powerplant from the auto wreckers. Maybe even
use the auto systems for FI and computerized engine management. I
want to get all my parts from the local auto store. I am drawn to the
earlier systems like the first really dependable packages like the
bosch systems in my parade of Volvo 240's or the outstanding record of
the 90's Toyota cars and pickups.

Direct drive would appeal to me more if oe could invert the engine.
Wittman did it with an aluminum Olds v8 I think. How feasable is an
inverted engine and the resulting dry sump problems?