KSC USP Compact broken blowback-ram+cylinder

[Goge]

So I'm posting this for a buddy of mine. I'll also be trying to fix it for him, seems he's given up on it. First the picture because it explains everything.

As you can see the o-ring is missing, I can't seem to find it's part number in the manual, and I've tried looking for listings of o-rings they use to find the proper size but its all Japanese. My buddy actually owns a hydraulics and pneumatics shop and has tried every standard oring he has that fits, or should fit, but none have worked properly. In the process of putting together and taking apart the gun to try all the orings he has now lost the only remaining spring that was pulling the cylinder back. One of the pegs (see C in photo) has been snapped off by a previous owner as well.

So how can I find out the exact o-ring I need and where if at all can I buy replacement springs?

[Kwokwai]

IIRC the springs are the same ones as used in another model... the M9 Beretta I believe. Perhaps somebody can confirm?

[Goge]

Thanks Kwokwai, I've found a place to buy replacement springs, 4.99 before shipping. But I don't see replacement o-rings on any site that sells gbb parts. I guess they assume you can go out and get them locally.

Does anyone know what size it is supposed to be or how I can figure out what size it should be?
It's part 28 and is listed as Main Piston Seal which isn't very helpful.