Yeah I needed to touch up the holes on a MVVS muffler a little and elongate them slightly on the inside as the FOX engine's holes are about 1/2mm closer together. Depending on the muffler and how large the holes are, you may or may not have to oval the holes slightly. I put a Bison in-cowl muffler on my Fox 45 powered Japanese Zero plane, and I had to elongate the holes on the inside slightly to get the screws to go in. But another engine, one of my .60 engines, the MVVS muffler fit it just fine as is.

I am not sure the carb would have made a difference. it is interesting you couldn't go WOT. it might be the smaller prop you used versus me running a larger prop.