First, I've never owned or flown a Sig Hog Bipe. So consider my advice with that in mind.

I doubt that there would be a serious problem with using an OS 91 4 stroke. I would suggest, that to be safe, you add some reinforcement to the firewall.

I would imagine that the 91 would give you excellent vertical! You might want to back off on the throttle when pulling out of loops or dives though. No sense ripping the wings off.
Dennis-