I am going to go against the grain here. I say that the 65 Sportster is a bad choice. Do not get me wrong I like 65 Sportster's. They are an under appreciated, under used engine. They are light and fairly powerful. But they like a real big prop. They do not take well to a short prop at high RPM's. My feeling is that you will run out of ground clearance. And the Sportster is kinda wide. You may have to do some major surgury to get a mount in there to fit the engine. If you must use the Sportster try the built in mount. They have (or had ) a mount built into the back of the rear cover. I would try an OS or a TT or even a new magnum. I believe the mount holes on all 3 of these engines in the 40-45 size are the same. Good luck with whichever way you go. BTW I feel the Sportster wiil not be too powerful for the plane. Just some size issues.

Mark Shuman