I've had 3 Hog Bipes and all I can say is the plane WAS designed for a 91 4-stroke. if you put that huge 120 on it you're gonna have several problems. first off is weight, you're gonna have to add a ton of weight to the tail to compensate for that big engine up front. second off, all that extra weight is gonna put a lot of strain on the landing gear, beef up the the area around the landing gear in case you have a less than perfect landing. The hog bipe wasn't made to 3D, so you don't need a motor that big on it. A 91 4-stroke has all the power that the Sig Pig will ever need. If you just absolutely have to 3D a biplane, then don't get a Hog Bibe, but rather get an Ultimate Bipe or Chip Hyde's Double Vision because those ARE designed for 3D.

just adding my .02 cents worth