Fit the engine in the plane first W/O cutting for the muffler and pipe.

THEN once you have everything ready, start worrying about the muffler and pipe.

I'll bet you'll get several ideas on how to go about the rest.

Since you did not cut the cowl for the muffler, those options will remain open to you.

Remember that the Magnum uses the same sized piping as the OS so you have a lot of possible choices.