I sent these pictures off to Tony a few days back when he had a question about a leaky petcock. I'd run into the same problem with the MSS fuel taps a few winters ago.
I remember Floyd G. telling me that he would use a wine bottle cork to repair his petcocks and clutch pack inserts. Having at it, I pulled the small screw on the side of the body and withdrew the cork/piston valve assembly. Floyd had the earlier 40's style fuel taps (push/push) with one side having a hexagon. this type uses a flat section of cork. Tony and I have the round Ewarts style.
Ed G. had the round type corks for repairing leaky Ewarts taps. It would be interesting to find how to cleanly cut a cylinder out of cork material. I've replaced a several corks in the past few years, mostly from draining the tank dry for winter storage.
Ed's suggestion was to use a punch and knock the brass riveted end back into the pull knob. From that point its a matter of adding the new cork section and tapping the brass back into the knob. There seems to be enough material to do this a few times. If your handy with a small lathe you could make a new piston section that threads onto the knob.
On a side note, Tony found that you can boil the cork piston assembly to get back some life. I've also removed the valves and stored them in a small container of gas when the tank is drained.