Hi Carl...For a WD M20 the gearbox sprocket is 18 and the rear sprocket 42...
Civilian models had a 19 on the gearbox, the WD model having the gearing lowered a little for convoy duties and off road work.
If you want to alter the gearing the easiest way is to alter the engine sprocket, though the options are strictly limited due to the low power output of the engine....Ian