Clean it up
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--I'd dismantle the weapon completely (here's where a set of real gunsmith's screwdrivers comes in handy) and scour the whole gun with 000 or 0000 steel wool and plain old vinegar, which will help clean it up and slightly etch the steel, bringing out the pattern in the damascus. I would not use any stronger of an acid.
Take the other hammer to a machine shop and have them duplicate it in mirror image form. Try to get some springs and stuff from Brownells or some other supplier and buy a cheap stock that closely matches the gun and carve it to fit.
The gun may still be suitable to shoot if you shoot low-load black powder (or a black powder substitute) loads, but that's your call. Smokeless powder will blow the gun, even with very low loads of smokeless, because it burns differently than bp. Smokeless has a high spike on the initial explosive burn, which will weaken old guns like yours until they pop, which of course could happen on the very first smokeless shot. So, don't shoot smokeless powder in it, ever.
You can buy a very inexpensive shotgun shell Lee reloader from Cabellas for under $40.
Wolf Lies Down