Skip, I've just finished one of these and built it with a GCBM R/C 25cc gasser to save some cash on the glow fuel prices. The recommended prop for this engne is the Top-Flite 18x6-10. There's plenty of ground clearance and is ideal for this engine and airframe. I used a "Pitts" style muffler and the entire engine is completely enclosed except for a cut-out to choke the carb.

While the 25cc is at the bottom of the sizes listed by Dynaflite, it seems to perform just like the full size plane. The Decathlon was never intended to do 3D (who'd want to with it anyway) but can easily do the Sportsman Catagory with style.

Here's a couple of pictures for inspiration.