RE: Larger Revolver
Yes, I think the .91 4-stroke will work. With the original revolver, a lot of people said a .46 2-stroke wouldn't be enough but it is. Your engine should be just fine for sport type flying. It isn't intended to be a 3D airplane. If you watch the GP video I'm sure it is flying with one of the recommended 4-strokes, likely an OS .81 or .91. Of course more power never hurt so a YS-1.10 would be a lot of fun too.