A good restorationist can find an original percussion lock very close to your mortise size, and tweak it to fit your gun. It's easy for those guys to move the cutout for the percussion drum to a modified location to fit your rifle, and your rifle will have better value if you use an original lock, slightly modified, to fill the mortise, rather than a new one that will always stand out as a major alteration to the rifle.
It's awkward to recommend anyone to do the work unless we know where you live. It's a lot easier to do this kind of repair if within driving distance so you can meet face-to-face to discuss the work to be done, get good recommendations and feedback from the gunsmith, discuss any issues you may not have been aware of, etc., and agree to what will or will not be done, plus get the price "ballparked" so there's no surprises when done. And it sure beats packing, insuring, and shipping a gun back and forth, with the risk of damage, loss, etc.
Shelby Gallien