Having looked through my 1936 -57 Lucas catalogue - the only bike listed with an MN2 magneto was the '38/'39 Triumph 5T. A model MN2E was fitted to '37 - '50 Scott twins, and an MN2L fitted to '49 - '57 Enfield 500 twins and '53 - '57 700 'Super Meteor' twins.
The more readily available MN2L from a Mark series Douglas may provide the basis for your mag, but will need the ears (for bolting to the back of the timing chest) removed to provide the correct appearance - also, the rotation may need reversing.
Hope this helps,
Regards,
Eddie.