I've finally completed the muffler.  The final step was enclosing it all in a .020 stainless shield.
I'll divert some intake air through it in an attempt to push as much heat from under the cowl as quickly as possible.

The final exit is 1.5".  Restrictive, yes, but I'm counting on it for significant noise reduction.

The cardboard/pink foam construct just above the muffler is the nascent form for the intake air filter box.

The 3/8" aluminum tube running along the bottom front of the engine is the emergency alternate fuel supply.  It ends in a piece of fuel line that connects to the bottom of the BMW throttle body.