Inside diameter rear sprocket

Hi all 
I'm about to replace my rear sprocket on my KZ650 1977 for the second time in 12 years (i do very little mileage) the last time i purchased a chain and sprocket set, i found the inside diameter of the rear sprocket was too small so I had to get it machined to the correct size. So my question is which is obvious are there different rear sprocket sizes concerning the inside diameter. Let me put you in the picture my Kwak is a 1977 model but the first owner (in Berkely California) changed the front and rear wheels to alloy ones from the original spoked version. Is this where the discrepancy is coming from, as I'm not sure what year wheels these are.  Any help appreciated 

650b models had spoked wheels, 650 c models had mag wheels. Both are 77 models.
You Nedd to determine which mag wheels you have as if aftermarket they could be different again. 

JT Sprockes web side shows the dimensions to every front and rear sproket they made.

www.jtsprockets.com/catalogue/?searchtype=buyers  

Pics for the (K)Z650 530/630 chain typ sprockets.