I'm trying to get a muffler to fit inder the cowling...It's a warbird and the engine is appropriately sized per the manufacturers recomendation. IE .60-.90 engine, and it's an .80 2-stroke, the giant mufflers that come with it don't fit at all and I'm used to using pitts mufflers, but this one is even smaller than the std. Slimline (it's a rather small JTEC, strap-on) and was supossely recomended for this size engine.
I know that bigger tuned mufflers increase rpm & performance, but I'm wondering what effect the small muffler will have on my engine as a means to keep the streamlined cowling pretty.
I'm just concerned that my engine may not like the smaller pitts and what that may do to my prop selection.
Any thoughts on performance, etc???

Small mufflers make them run hot sometimes

You will probably not notice any difference but inside the cowl you will need more exit for the air flow or it could get hot. Most of us cut a lot out of the bottom of the cowl for this reason. May not look as good but they tend to run better,

w8ye and Grey Beard are correct. 2 Strokes like to breath and don't like any type back pressure. A smaller muffler will make it run hotter.