I have made wad pulling screws by digging through an assortment of wood screws to find an appropriate one, cutting off or turning down the head and then threading the shank. The cap and base piece are straightforward lathe jobs. If diameters are such that standard thread sizes can be used, it is easier to do.
With respect to baleen - years ago I could have acquired a piece from the first bowhead whale taken in the Eastern Arctic when whaling was restarted. As it happened, I armed the grenade fired from the whaling gun used to harvest the whale, and had an invitation to go on the hunt. Couldn't do it because I had non-refundable tickets to leave for our holiday.
I think that a ramrod that tapers right down would work for shotgun use because less force is required that seating a patched ball.
I, too, enjoy working on antiques at least as much as scratch building. The challenges are more varied. And nowadays it would appear that a manufacturer's license would be needed to make a cap gun, although longarms with touch holes are still OK.
The lock on this gun is signed I Whitehouse. Tiny London on top of the breech. Post 1813 Birmingham proofs on the bottom of the barrel along with IW (Isaac Whitehouse) and Stubbs.
Sometimes guns were returned to the maker for percussion conversion, or to another gunmaker, rather than a gunsmith. Not unusual for the conversion to be a remanufacture, rather than a simple conversion. Knew a chap who had a cased Joe Manton 20ga made in 1812 than had been redone. If Manton's shop didn't do it, it was a shop capable of work of the same high quality. New patent percussion breeches were fitted and the locks were reworked to suit. Beautiful little gun.