You can cut the header portion, gut the baffle and build a slide in convergent cone to good effect, but it's less a matter of equations and more a trial and error thing, mostly due to the fact that you can't really shift the header length or divergent cone position, just the convergent. While that will still have an effect, your dwell section will always be weird.
I'm sad to say I just scrapped 3 stock Puch pipes, I would have said come grab them. I'm in CT too.