norbe, unless your model is over-built (i.e., heavy) power won't be a problem. The model will fly on a .46, so a .61 is plenty. The .91 two stroke, unless you just want unlimited vertical, etc., adds complications; you need a bigger tank, the engine is going to vibrate/stress the airframe more, etc. Plus, the almost-mandatory Pitts muffler will be larger; and you don't have much room in that cowl at all.

I don't like Super Tigres, but that is purely a personal observation. You do know they are made in China now? And I didn't like them when they were made in Italy, either...<G>

a340's suggestion of a Saito 82 might be the answer for a four stroke. I haven't seen one yet, but by all accounts it is a typical Saito (i.e., good) and should have plenty enough power. If I recall, the Saito 91 is a bit smaller than the OS .91 Surpass; so something that size should fit fine. For an idea of just how big the Saito 100 is, check out the photos, in the ARF forum, of the Pacific AeroWorks Edge 540 60 size ARF. It has a pretty big cowl- MUCH bigger than the SkyBolt- and still needs two holes cut in the side to clear the rocker arm covers.

On the subject of power, we tend to become spoiled by sport planes that have a power-to-weight ratio far above one. IOW, we think we NEED the ability to go straight up forever, leap off the runway in ten feet, etc.

The Sig SkyBolt hails from an earlier day of model design; it is intended to fly on its wings, not the engine. It may do well at the ever-popular 3F (Flip-Flop Flying) that seems to be all the rage these days, but consider this: it is a very complex kit, requiring many hours and much patience to build. Why risk all of that with gyrations and antics and attempts to emulate a helicopter??? Buy one of those electric foamie things or a profile fun-fly plane if you have the urge to flip-flop around, and save the SkyBolt for days you want to FLY...

Anyway, my point was that overpowering this particular model might cause more problems than it solves. Just my opinion; and I'll get off the soap box. Sorry...