My Hog Bipe flies beautifully with a TT .91 four stroke on it. I wouldn't want any other engine. A friend also flies his Hog Bipe with a Saito .91.
For more info on what engines people are using and lots of pictures of Hog Bipes go to
www.fubarhill.com/ and check out those planes.
I personally think anything less than a .91 four stroke would be a waste on this fine flying airplane.
Enjoy.
Jim