i dont have a parts manual for your bike, however the one on my bike is 3/16 inch, I would guess yours is the same
you would probably get one from a cycle shop (halfords) ?
is yours missing, i dont know how you would know that especially with the engine in the bike where you cannot tip it to one side?
again...on my bike the steering bearings are bigger at 1/4 inch, as i said before I would guess yours are also