Hi Jack
The spokes on later Baghira and Mastiff are stainless. BSA-Regal supplied me with replacement spokes when I broke two in the rear wheel but they only have plated (not stainless) spokes in stock as they have not had to re-stock with the latest, stainless ones.
I don't know where you are in UK so would recommend Central Wheel Components in Birmingham
http://www.central-wheel.co.uk
or Alf Hagon in Essex
http://www.hagon-shocks.co.uk
I do my own wheels, quite possible if you are careful, make notes as to spoke pattern and offset as you go, and has the advantage of being able to 'lace' (loosely build) the back wheel and then fit it into the bike with no tyre on, fit the front finished wheel with no tyre on and then get the wheel alignment perfect by adjusting the offset of the rear rim during trueing (final tightening of spokes, getting the wheel round and straight running).
Try it all on a bicycle wheel first and then on your MZ!