It depends on your power expectations. As I see it, For a .91 four stroke to give really good performance, you will either need to have an EXCEPTIONAL .91 four stroke, or a VERY light CG Ultimate, or both. The problem I see is that given the marginal quality of the wood in the CG kit, it is HARD to build one really light. I have flown one with a YS 1.20 and also a friends with a 1.08 stroke (ASP not O.S.) and they BOTH were unlimited. However, the 1.20 four strokes gave more low end "grunt" (instant response) while the 1.08 had just a little lag while the RPM's came up. The 1.08 probably had the edge on maximum straight-line speed though.

Have fun,

Lee