You can switch to HFC134a (which replaced Freon 12). It has about the same pressures and tolerances.
As for worth? That I don't know. I'd assume as a collector piece, it'd fetch a pretty penny, but as a fieldable piece, there's probably better items (mags? parts? etc.). It's a toss-up. You'd have to find a collector willing to pay a premium above a similar gun's price for its collectible nature. Rarity also factors into it as well.
I'd look at what a similar gun will fetch now, how rare the piece is and play up its collectability if you plan on selling it.
Last edited by Firewalker; July 10th, 2011 at 03:00..