I am building a Sig four-star 120 and was concerned about using an OS FX .91 engine as it is at the bottom of the recommended engine range. I called Sig directly and they told me that an OS FX .91 would fly the plane just fine, as long as you are not planning on tearing up the sky. I have also built a Sig LT-25 and used the lowest recommended engine (an OS FP .25) and it flew just fine as well. Extra power is nice, but if it is the difference in flying what you have and putting out several bills to buy new, I'd say fly what you got.