A nice addition to a running bike is the use of original spark plugs.

Two distinct types, KLG KS5 and M80, are given in the specification pages for both BIG4 and 16H.
The types are fysically quite different, as can be seen in the pictures. 
The M80 (early and later version) is made with a Porcelain isolator body, while the KS5 has an old fashioned appearance with a Mica isolator.
Both types were used during the war as can be seen on a 1942 Army Motorcycling Film on the Motorcycle Maintenance.
There are spark plugs found which are provided with the "WD broad arrow". Whether this was standard for all plugs is not known.

If you can't find an original, but still want to ride, the following modern day spark plugs are technically equivalent: 
NGK A8, Champion D6, K7, K8, Lodge 2HV. The threads are 18mm x 1/2" reach.

Another nice period item is the John Bull rubber sparkplug carrier as shown at the bottom. It is not known whether these have been used in large numbers or by army motorcyclists. They are not mentioned as motorcyle accessories in army documents I have.