Hey rickevin, this is a gret plane indeed. Any 2 stroke smaller than a fifty somthing is going to be too small. I'm running an OS .61 FX in my UCD 46 and it's just enough. It would barely hover with a 13x4. had to switch to 13x5. That did the trick. but even then I don't have a butt-load of power to pull out. it's good though. This is without header and pipe, just the stock muffler with the baffle removed. I've heard that an OS .50SX-H (heli engine) works really good for this plane. But that's about as small as you can get. A regular .50 size engine won't pull it good enough. I don't see how GP could recommend a .46 size engine for this plane. it weighs over 6 lbs and has a pretty good length and span.
Hope this helps.