Never gutted a viper. If it's an actual 007 instapierce then I may have a face seal that would work. I did make a punch for the nelspot ones and made a few urethane ones that I used in Nelspots with good results but I never really followed through on that one.
The CCI ones are close to that size but because of the hardness I've had mixed results. If it's a CCI or other (BE?) instapierce, then a CCI seal is probably the best bet. Either way, they're not expensive and always nice to have on hand. If the rest is normal sheridan guts, I do have some obviously.
The setup on that gun confuses me. So CA is going into the valve and you're using a spent 12g to seal off the front of the valve (where the instapierce is?).