I love four strokes but agree that using what's already on the shelf makes a lot of sense. And, it's easy enough to swap engines later if wanted or needed.

To the question about the Magnum 61 four stroke. My brother has an Ultra Stick 40. He started out with a TT-46-Pro. He swapped to a Magnum 61 four stroke hoping to improvethe performance of the plane. The engine runs great. Low idle for as long as you want, great throttle response, no deadsticks other than staying up until out of fuel. Performance-wise he considers it to be the equivalent of the TT-46. It turns a bigger prop, but at much lower rpm.

He had a 70 four stroke in the box so he installed it and is finally happy with the plane.

So, if you decide to go 4 stroke, the 61 will give performance like a pretty strong 46 two stroke.