On measuring the exhaust port diameter won’t a port map give you the chordal port diameter by measuring it flat on a table?
Arc or chord width doesn't matter as long as you know which you have. Software converts that to the appropriate dimension. Some use Chord and others use Arc. We have the technology.
Most software uses Blair formulae or versions of it and few of them seem to create an exhaust that works for some odd reason. One reason is that they all make assumptions as to exhaust gas temperature and they may be way off for a particular motor. MOTA calculates exhaust temperature all down the pipe and shows it as a moving wave which is really neat.
You could also use EngMod2T which is even more complex.
Just a thought but if that motor is reaqsonably similar in some respects to say an H2 big cc motor or say another 750 triple or close to say an RM or CR250, that might give you some ideas as to what to aim for and could give you a place to approximate the pipe at least as a starting point.
Making a pipe is maybe the easiest part of this exercise as long as you can think in 3D
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