My parents were in Montevideo, Uruguay, so I asked them to keep an eye for pens in the flea markets. Unfortunately for me (though fortunate for the country's economy) the cruise ship visitors have quadrupled and the flea market prices for those not local have followed. They did manage to find a functioning 51 (and how sweet it is) for less than $20, and a few 51 copies with interesting nibs. Only one is not in working order, labeled "vis pen" on the interpreted arrow clip, I attach these photos in hopes that it can be repaired with some sort of soaking and rubbing combo. I don't really know. I predict I'll be visiting Berliner Pens pretty soon, or leaving it unrevived.

I'd start off with a nice vinegar scrub using a soft toothbrush wouldn't. It's a cheapie, so the worst that will happen at the end of the day is it still won't be working; I don't think I'd suggest any kind of toothbrush otherwise. I've got a couple very similar to that, and as far as I can make out the reservoir is glued to the section, so further dismantling is nigh-impossible. Try alternating vinegar and the usual 1:10 water/ammonia suggestion here, with water rinses between them in hopes of avoiding corrosion. Hopefully after the first cycle, you'll be able to work the filler and get the inside de-gooped on the strength of that being one of the few piston-filler types that you can really generate some pressure with.

edit-- it's probably worth the effort, too. The one I've been been able to get running (no india ink in it) has one of the more interesting points on it, although appallingly flimsy.