An IO360.  Now comes the fun part.  For me anyway. 

About 15 years ago I was looking at a 200 hp, angle valve engine in a Cheetah/Tiger and 'by damn' it looked big.  I talked to the owner and he told me that it would do about 149 knots true airspeed.  My Cheetah, at the time, would do 140 KTAS so I wasn't that impressed.  When I found out he needed to fabricate a new inlet, modify the engine mount, create new baffles, and on and on, I knew there was a better way. Enter the 180 hp parallel valve IO360. With a LyCon prep, this engine made over 200 hp on the dyno. Since it's 40 lbs lighter . . . . . it's time to go hunting.