Air leaks were sealed by using some of the inner tube where I got the tire valve from. Surprisingly, two layers of the stuff gave a perfect seal. Here's a closeup of the seal. It was sealed across the bolt slot and in rings on the ends of the loading slot on the smaller pipe.
Closeup of end of air tank with new tire valve installed. Tire valve was cut out from an inner tube.
Full view of the gun. In all its glory. :D
One thing left to do: mount the bicycle pump below the Unknown breech and connect to tire valve. That screw connection that connects the air tank to the hose nozzle needs to be glued up, too -- that's as leaky as the sound-proof practice rooms in the NBTHS music department.
Ciao for now.