I don't know of anyone who carries drawings of a Hatfield rifle. Track of the Wolf, Muzzle loader builders supply, and possibly the Log Cabin Shop do carry drawings for some rifles, as well as selling precarved stock blanks compatible with those drawings that they do sell. I suggest checking out what those suppliers offer.
Another concern is the near impossibility of finding the lock and furniture for the Hatfield. I don't know of any supplier that offers parts compatible with the Hatfield.
The "Hatfield" rifle, was, and is a fantasy gun, that is, IMHO, ugly as all get out, and doesn't fit all that well.
I suggest that if you want to spend the often considerable time and expense to build a rifle, build something with style. A Southern mountain rifle, A NC style rifle, a PA rifle, or VA style rifles have tons of style over the Hatfield. They are far more enjoyable to shoot, and retain much more value than an ugly "Hatfield" styled fantasy piece.
Just kinda thinkn'...typn' out loud, so to speak...type.
God bless,
J.D.
