That shouldn't be all that difficult. You'll have to find the size of the thread and then purchase a tap for it. A comprehensive hobby shop (the sort that sells model airplane engines, not the kind that sells artificial flowers) may be able to help you find the thread size. Or an old-time hardware store, though that's a stretch for very fine threads. Find a tap at eBay.
Now if the, uh, male thread is bunged up you may be able to improve it by chasing it with a small, fine 3-corner file. Harbor Freight Tools has their 'Swiss files' and that's the sort of thing you need.
Any idea what might have happened to the threads? Corrosion can be reversed or removed, sometimes. What are the materials?