I would strongly recommend sticking with a 2 cycle on your decathalon. I owned one a couple years ago and had a saito .65 on it. It was so torquey and the tail moment of the plane so short, i almost crashed just trying to take the thing off. After flying models for ten years prior, i'd be surprised if anyone less experienced could've handled the four stroke torque on that model.
Switched to a 2 cycle and while i wasn't estatic about the plane, it performed better. Ground handling was still pretty squirrelly, but i didn't have to deal with the added problems of a big prop up front creating too much torque.
I hope you have more fun with your decathlon than i did mine, I put it up and haven't flown it since that season.