Both of these firms do wide ranges of investment cast hammers, for muzzle- and breech-loaders. I helped an American correspondent to find one for, of all things, a Calisher and Terry capping breech-loader. You are unlikely to find a close match (unless you are parpared to do a lot of filing), so you will probably have to replace the pair. They do need to be finished and have the tumbler square drilled and filed.

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