They originally wanted to use GURPS, but something happened with that. Not sure what, and too tired to research it right now.
It uses it's own system, but is
VERY RPG based, so it wouldn't be any real stretch to make it a Pencil and Paper RPG.
There *WAS* a fan-made one that got C&Ded recently, IIRC.
And, ya, I'd buy it. And Demo it as well! I loves me my Retro-Post Apocolyptic Games!