Campy 88/89 rear derailleur b screw

Fixing a friend's old Bianchi beater that has 88/89 Athena parts and a bad rear derailleur. Campy rear derailleur of this era do not use B screws but a toothed ring which breaks very easily and mine is broken (and a spare der I had also has the same part broken). More info about vintage Campy der can be found here at retrogrouch.
But I wanted to know if anyone knows a fix (even a kludge fix) or if any more modern Campy derailleurs are compatible with Syncro shifters. Note these are 6spd and utilize a slant parallelogram design unlike the earlier derailleurs and my friend would like to keep is all Campy if possible.