The wheel sheave can mount on either side, no issues there. If it is the original 1937 sheave an A belt is correct, if after about 1940, it will take a B belt. As for pulley diameter, Simplex used 5” and 6” on the shifter bikes, but with the Villiers motor, I think you could use a 3” pulley on the jackshaft without issue. Belt length will have to be determined by measuring when everything is mocked up.
BTW, A 4L 1/2”
B 5L 5/8”