I don't know of any parts source for those guns. If you are heir to a fortune and cost is no object, you might try to find a gunsmith who would tackle the job (lotsa luck!). Pepperboxes are pretty simple, but the gunsmith would first have to figure out what parts are missing and what they should look like, then make them on a trial and error basis, a very time-consuming process. I am pretty sure the cost will be a lot more than the gun is worth. Another possibility is to try to find another identical gun and make one good one out of two, but Marson made several models and I don't think they are the same internally.
Dixie Gun Works has an Allen Pepperbox kit; it would not be the same and I would be very surprised if parts were the same, but if you have the money it might at least help you with the internals of a pepperbox,
I wish you luck, but I think you may very well end up going the display model route.
Jim