For the benefit of those folks following berg's problem, turns out the gear in question is the fibre magneto drive gear, not the whole gear train. He's gotten an ATD that won't mesh with the BSA cam gear. Turns out the rogue ATD is from a Triumph, with 50 teeth, not the correct 44.
Took it on myself to send a personal message, and with a bit of detective work looks to be sorted.
Time for bergs to celebrate in the usual way?
Swarfy.