The first thing you wanna know if these regs are still being made and used in modern guns. KEE must have bought the rights to its design when they bought WGP, cause this exact reg is used on the Empire Sniper and Resurrection. Their rebuild parts should work perectly if you wanna go that route. PBGateway also has original WGP parts if you'd rather have those: WGP Inline Regulators & Parts
The reg is pretty simple, with only 2 orings and it's seat. The oring around the large piston is a 112 and the oring around the smaller brass piston is 011. The reg seat can just be flipped over to it's unused side if the reg has never been rebuilt; no need to buy a new one.
The 011 oring might actually be 010 or 012. WGP slightly changed the design of the reg over the years, so if you have a very old one, you might have to use a different oring, prob 010 IIRC.