You can use any engine you want up to a 91 four stroke. With these old designs which were set up for .60 two-strokes there is a real "squeeze" factor. Also many of the old designs carry lead in the tail to balance the plane. I can't see any reason not to use a smaller engine. If I had suitable engine I would build the Troublemaker. It's about time a really good stick wins with the smaller setup. It would set a good precedent and make more kits useable.