My dad built one several years ago and put a YS .91 and it was way more than adequate. It was fast, it would fly slow. It had no bad habits and the engine never flamed out in the air, I mean never! It took just a bit, an Oz or two of lead in the nose to balance it. He had heavy Century Jet retracts in it, so weight wise I think he was 12 or 12.5 pounds. The current YS in that size range is the 110, which would pull the wings off of it!!!
So a Surpass .91 pump, or YS 110, Saito 100 would all be great choices. I would stay away from the magnums.