After some research, I learned that the little bottles the paintball guys use have variable pressure regulators. They are designed to hold up to 4500 PSI and regulate down to 800 psi which is CO2 pressure.
I picked up a used 13ci bottle with a regulator and had my father machine an adapter from the point the old CO2 cylinder screws on to the bottle.
This replaces the CO2 tube, on the angle.
It took some effort to line up the treads, but the foster filler is on the side, the gauge is on the bottom.
It took a little work on the stock, but it all fits together even better than expected.
It's punching _very_ clean holes in the bad NRA targets, so I have a feeling that the CA is pushing these pellets out a lot faster. I need to get a chrony to get it back to target speeds. Or it's quite a field rifle at higher pressures I suppose.
Anyway, quite a capable and accurate rifle! The kids are going to really enjoy this one for a long time.
Oz