These engines are more than 13HP but has anybody heard of anyone
powering a plane with a Kohler V-twin?

These 2 engines are the top of the line for both of their series but
look quite workable. The Air-cooled 40 HP engine could loose the forced
fan cooling and probably drop 7 or 8 pounds... The water-cooled engine
only has a dry weight listed and the specs don't say how much water it
holds...

http://www.kohlerengines.com/onlinec...?engnID=147160
http://www.kohlerengines.com/onlinec...?engnID=180360

Anyway I found these engines while looking for something to power
another project.

Tony