Tom

Depending upon what material your motor mount is made of, and how much material you would need to remove, you can simply grind away some of the mount until the muffler will fit. Great Planes actually suggests this in the manual for one of their ARFs that I built. That mount was made from a plastic-like material so it would have been easy to grind it.

Obviously, you would not want to make the mount too thin or it could fail. Also, make all your cuts with a large radius because cracks will tend to start at a sharp edge or corner.

Sandy T