Beautiful bike. Coming together well. If u haven't figured out your exhaust for the rectangle port to expansion chamber pipe shoot me message I can most likely help, IF the ports the right size.
You may want to look at some aborit forums, the saws they build dmostly have the esquarwd porta and they build the pipes around that. May consider making a small manafold, and use that to change the shape to fit a pipe then clamp or wdl your custom pipe up.
I love building those expansion chamber pipes. Not easy, if you do it all by hand like me. I do all the math on paper, then draw out the templates. Weld the pieces together then use a pressure washer to expand the metal to the ecorrwt shape and size. All tahts left thn is I pie cut the curve to the manifold. Remember to taper that heder pipe.