PM me if you need more exact dimensions. The muffler opening APPEARS to be too wide, but that was needed to allow the cowl to be installed WITH the muffler on the engine. The angle of the cowl front/engine hub down to the rear-most exhaust tip is what drives that. Difficult to explain, but hope this helps.

FWIW - Carving on a cowling is also WAY DOWN on my list of "things I enjoy".... also.... It seems just so WRONG to do....