Well if it were me I'd cut off the lower plug flush with the wide part, drill it out, make a new pivot plug that has a smaller diameter to fit through, make that plug, and fasten with a screw and washer. Sort of like this:
http://anotherairgunblog.blogspot.com/2008/03/making-barrel-bandpivot-plug-for-1377_29.html
(Scroll down to the last two pics, which show the assembly I'm suggesting)
Obviously not as beefy or as much of a departure from original...
But it would be nice for it to just look original? You could perhaps make the repair with JB Weld, etc...binding it together with wire "rebar".
Or...I don't know. That sucks that it broke.
Nick,
From ruralburbian Philomath.
anotherairgunblog.blogspot.com