Chris,
Vehicles made after about 1935 (ish), require a stop light by law, also the old WD tail light does not have enough area of lens to be legal.
The way I got around this was to retain the original WD tail light, there were no number plates on military motorcycles after about 1940, just census numbers on the tank. A tac sign was painted on the rear mudguard below the lamp, about 3" square. I then made a plate up, to carry a modern number plate and Stop / tail light, which has a reflector combined (1950/60 Triumph, BSA, etc type are good one). This plate hooked over the rear of the carrier and was secured by the big wing nuts on pannier frame. A small plug on the cable from the lamp, pushed in to a socket under the pillion seat. Then when on shows or displays, the rear number plate can be removed to reveal original lamp.
Richard