The parts will need to be made and the hammers are the costly part. You might get lucky and check with some of the Brit supply houses such as

https://www.peterdyson.co.uk/

https://www.blackleyandson.com/

The firing pins and retainers are much simpler to make. Henry made a tons of styles and sizes but your not the first one to want to put one right either. If you get the metal done there are several good engravers that are members of this site.
Steve

http://www.bertramandco.com/
Booking African hunts, firearms import services

Here for the meltdowns