You can use the stock muffler. The model kit manufacturer may refer to stock mufflers as "sports" style because they are not specialized mufflers for scale or high-performance applications.

Basically, if you can put the engine with the stock muffler on the plane, and the looks aren't critical, then you're fine. You may have to relieve some of the cowl, though, to fit the muffler, but that's extremely common.