Most thread sizes (including this one) are shown in the WD Spare Parts Lists - Part No. 3403 (original price 6d - 2.5 current pennies !) is 3/4" x 20 tpi. which is a standard BSC (cycle) thread.
Most dealers in spares for older English motorcycles ought to be able to supply.
This is the one (with the post-war - 1946 part number but cranks were unchanged)
