Hello; Another vote for the four stroke engine. The 4 star 60 builds a bit tail heavy, so a heavy four stroke will work great. I started with a Saito 80, but I had to add weight to the firewall, and I had an old OS 90 FS that was heavier that I put in there. It worked so well I left it in there. I think the surpass 91 is a little lighter then the FS90. You'll find that the four stroke flies at part throttle beter then a two stroke can, and if you fly like me, you'll be flying at part throttle a lot more then full throttle.