Right, the K3 motor is the same unit, motor and worm, part no 276-005 on diagram E3227-IS001; which applies to all my K3's, catalogue number 32- 275 to -278 inclusive. Worth trying Bachmann to see if they have that one! Remember to lift the motor out from the worm end, and install a replacement brush end first...
Also checked the V1/V3, but no good, 30mm long motor.
There are probably other Bachmanii with the required motor and worm combination. The exterior motor dimensions correspond to the Mashima 1426 'flat can' ; 14mm over the flats, 26mm length over the 'can', and the worm is 5mm diameter, 10mm long. I reckon the BR std 4MT 2-6-4T, G2A and Robinson O4 as 'likely suspects'; don't have the first anymore and foolishly didn't keep a copy of the poop sheet, (because at the time Bachmann made all their poop sheets available on their website, a useful feature now discontinued) and it will now be tomorrow before I can look at the latter two, should Bachmann deny possession of K3 motor spares.