First I’ll thank you for rescueing a historic frame…

You don’t mention the R value for EPS, my preferred core for SIP’s. This is perhaps just as well, in that for R-37 the buns would need be of a thickness which would be proportionally unpleasant, seem out of scale and unappealing to the eye.

But yes, a urethane core panel is to be had and would work, though the price of panels is in a hyper climb at the moment, due to the current volatility in the OSB market.

Your greater issue is that your roof sounds as if it is far too underbuilt to carry such a system and the snow loads your area sees ( I spent a winter hewing timber a town or two over and know your in that Berkshire snow belt ) I don’t have the GSL’s for your area at my fingertips, but don’t think I’m far off in guessing your in the 90 – 100 lb range.

Another guess, you may have to upsize your rafters and purlin plate.

Bear in mind this frame has never carried more than board sheathing and perhaps several layers of roofing as a dead load and the snow loads that fell on it in the past, you are about to ask much more of it.

You need to consult with someone who has done barn / home conversions in your area, or perhaps a structural engineer who knows timber.