Marc, your heart is in the right place, but as the guys mention, this one will never give you a return on your investment of time and energy. That said, there is nothing more satisfying than saving an old horn from certain doom. I suggest you challenge yourself a little to see if you really want to do this:
It appears to be silverplated from the photos and the black tarnish. Is this correct? If so, ask your tech to oil the hinge rods and see if they can be backed out or if they are fused to the keys as JayeSF mentioned. If they can be loosened with oil, a little heat, and some patience then pay only for this service.
Once the horn is disassembled, get yourself a bottle of silversmith's polish (Hagerty's is a common brand here in the states), a bunch of old T-shirts, cotton swabs, tooth brush, and bunch of good old fashioned "elbow grease". Flip on your radio or TV and start polishing. It will be tough, dirty, painful (you will poke yourself on those old springs a bunch) and will seem like a neverending task, but you will eventually get a good look at what this old timers really looks like under the dirt and tarnish.
If you can't disassemble the horn or can't invest that much time in it, at least do the inside and outside of the bell. Either way this will give you an idea of just how much work is ahead for this horn, and remember this is just the cosmetic stuff! You'll then have a better idea of what it will really take to get her going. I've done cosmetic work on quite a few horns then take it in to my tech for the REAL work. It feels great to uncover an old gem, but there have been a few that ended up needing too much. Those ones now sit in my garage on shelves.
If you get this far and still think you want to do this, I have a couple old c-melodys beyond repair and there may be a Martin stencil in there. If I so happen to have the keys you need (long shot) and you want to pay shipping for them, I'd be happy to pass them along.
I know I shouldn't be encouraging you, but it can be a lot of fun. Just don't get carried away dropping money into this guy!