First off, the feds put them in the "dangerous device" category, or whatever it's called, because of the bore size. I think. Maybe that just applies to rifles, but you should check with someone who knows. If it is in that category, it's handled like a machine gun under federal law. And check with your state.
Regarding plans, jeez, it's just a muzzleloading, single-barrel shotgun. You don' need no steenking plans. Just be sure you understand the difference between the lock dynamics used for external-lock cartridge shotguns and those designed for percussion-cap guns. The first bounce. The second lock when they reach the end of their stroke.
Some enthusiasts in the UK make replicas (yes, they were legal there long after they had been outlawed in the US). Or they did, back in the '80s. Good luck finding them.
-- Ed Huntress