I have that exact setup and it flies great, OS 61FX with a 13x6. A friend of mine has the same plane with an OS FS91 and we fly the two planes together a lot. The four stroke definitely has more power on verticals but my two stroke does very well. When we're flying side by side I have to cut back to half throttle so the four stroke can keep up. The four stroke sounds much better but I'm happy with the performance of the 61FX, it's got plenty of power for this plane.