[SvingenB]

D-Motor is starting tests of their 6 cyl 125-135 HP engine. Basically a 6 cyl version of their 4 cylinder engine.

http://www.d-motor.eu/nl/lf39-40.htm

It is an interesting engine. A flat head, water cooled, 4 stroke. This makes it very compact, while maintaining large capacity. Of course, digital EI and FI.

 

[SHIPCHIEF]

That is a neat concept. I think the power output is a little low for most RV applications. 3.9 Liter is a bit small. 5.2 Liter (O-320) is the standard. With a slight power loss due to being a flathead, 5.5L would be a contender.
I have a Thorp T-18 with an O-290. That is 4.75 Liters, and 135 H.P. @ 2700 RPM. It flies well and had adequate power, but less would not be satisfactory.