Chain and sprocket

Hi out there. I've got a 77 KZ 650 and it's in need of a new drive chain and sprockets, my dilemma is this in the bikes life many years ago
the original owner swapped the steel wheels for alloy ones and I'm led to believe that the mounting boss is a different size on the rear wheel. If this
is correct does anyone know the correct inside diameter of the rear sprocket I would need to purchase. For this KZ 
the bikes history is it's an import from California to GB cheers in advance
 

Phil at zpower.co.uk will be able to supply what you need and advise. 
It'll depend if you have after market alloys or khi ones fitted. 
There's quit a bit of info on ht sprockets Web site re sizes and spacing etc which might help you out.