Marlin-Ballard extractors. I went nuts last year trying to find one. (I still would be, but I discovered the right one mixed into my Stevens parts.)
Definitely Stevens 44 links of better steel. I had a guy out in California laser-cut some 44 link blanks for me out of 4130 pre-hard, but that's as far as I've gotten. Even got the necessary carbide tooling, but the mill is in an unheated shed.
Model 44s almost always need oversize link pins fitted. This involves reaming the lever, breechblock and link to take a shortened 5mm dowel pin. You could offer that as a service, perhaps including the stronger link.
Ditto for Favorites, both "94" and "1915" styles. There is no shortage of Favorites, even 120 years on.
A kit of trial links for the 44. like was done for the Ballard. I have found that 44 links' hole spacing varies by +/- .005, and that makes a big difference in how well the action locks up, unless you're going through the whole barrel-fitting routine. (Which only people with a substantial lathe can do.)
Stevens firing pins are well covered by Jack First and Wisners. Ditto extractors and ejectors.
There was a guy in Pennsylvania making replacement turret plates for the Lyman Junior press, with holes for standard 7/8" dies. No longer doing it, apparently. I see people asking about them occasionally. Part requires accurate indexing.
I'm sure I'll think of more.