Hmm. Well, it looks as if a sorta' POW stock was an option on a Nitro, so, that question of mine is partially answered.
Eddy,
I'm looking at your pictures, and thinking that is a pretty grim project. It looks like it would be easy to end up with a bunch of money in replacement parts, for a gun that might never be worth more than $200-$300. A lot would depend on how sound the barrels appear, how clean they are inside, and how many mechanical parts besides screws are missing, and how much you can do yourself, versus paying a pro to do it. It might actually bring more money to people looking for Nitro Special parts, you selling off the parts at an auction site to recover your investment.
Been there, bought the T-shirt, mailed the post card, done that. Think hard before you put more money into this gun.
Best,
Ted