For the same weight of an OS 61 FX, you can get an OS 91 FX and the price is not too different. See what I mean?
For 3D, the more power you have, the better it is. Of course the best choice would be a 4-stroker, but if you want to stick to 2-strokers, try a 91, or if you think it's too much overkill , look for an engine in between...