I had a rossi 53 with tuned pipe on my OMP. The horse power seems impressive, but it is at 18,000 rpm. Great for racing, but not for 3D. Also the rossie likes small props. 10" or so. To perform 3D you will want as large of prop you can swing without hitting the ground.

Constrictor,

look at the overall weight of the engine and muffler. The four stroke muffler is much smaller. When you do this the weight of the 63 is not that bad. Especially when you compare to a larger motor with tuned pipe like the rossi. Again, like I said before, the key is thrust to weight ratio period. Not HP, not, RPM's.

I have a buddy who has a ucd3d and he has a saito 100 on it. It is awesom. Another has a OS 108 2 stroke. This combo is not bad, but the satio hovers better. After seeing the 108, I doubt a Os 91 would be enough for 3D. If I get one, I would use the YS 91.