Having experience with three G/P Skybolts, the 61 would be just fine for fast taxi work. Not much else though. I found that the OS 1.08 worked out to be a great two stroke, and the YS 110 is a very good 4 stroke. Other 4 stroke manufacturers would require at least a 1.20 to get excellent vertical and aerobatic abilities. You will probably need the weight up front anyway.

As far as the Sig Skybolt is concerned, the 70 will fly it, but you won't get to do as much with it as you will want to. The 82 would be a better choice.