According to the Lucas parts lists a '51 should have magneto 42248E, and again, according to the lists, cb cover should be 485619, which is the plastic job with the kill wire coming off at a tangent, with a clip to secure.
The alloy dome is part 459037, attached by two screws through the ears, and I'm fairly sure was to be found on some Beesa twins, but maybe right at the beginning on A7s, dunno. Most commonly seen on AMC twins.
The bearing housings do vary, it's a bit of a minefield as duTch says. Manual/auto advance retard, plastic cover, alloy one with the ears and the button, screw-on big jobs later on, etc. Often, if minor mods are needed to the fasteners (screws where there were pillars threaded both ends etc), it's not so bad but it gets more awkward when the mag body hasn't got the tapped threaded holes in the right corners and so on. Hard to know without seeing exactly what you have there.
If you've got a good working mag, maybe best to leave well alone until you need to do something more major to it? The bits are available to create whatever exact variant is needed - but it can be a fiddle, swapping bearing housings and outer races in particular.
Depends how politically correct you want to be I guess!
Cheers.
