A few data points
I learned over the weekend that the flanges that mate the down pipe to the manifold are modeled after a flathead V8. I measured the down pipes I have and they measure 1-3/4" OD, so I would have to confirm this.
I also read that there were 1500 original sets produced by Horsepower Engineering (now Bursch Exhaust) in Pasadena. Perhaps someone there might have some insight.
I would think that the down pipes would be fairly simple to reproduce. The manifolds and the elusive motor mount present more of a challenge. There are a few samples of both available for patterns. Casting in cast iron would have to be done by a professional facility. I did a few parts many years ago in manganese bronze many years ago and wonder if that would work - even if only for a prototype.
Cheers, -Kevin