So Pete, do you think that rotating the upper barrel 90º to the rear and using a standard tilt down muffler that exits on the star board side would show promise?
I kinda like the idea, and as you said if that works (and why shouldn't it) then perhaps progress onto finding a header for it and a pipe - or try the suggestion I mentioned about using the stack of the standard muffler for a header. It seems to have an almost square outlet, unlike very rectangular ones on other motors, and should be able to be pushed into service.
I know that Brian Eather here in Oz would have a pipe that would work - pipe number 1 part number 2025.
Interesting stuff!