520 is a 'kit', so can I just use the OE....

chain and just purchase the sprocket? My bike only has 2k+ miles on it so the OE chain is still good.

i don't think so, i believe that the stock chain and sprocket is a 530 which is the width of the chain. if you put a 520 sprocket on with a 530 chain then i think there would be too much side to side room on the chain (if that makes sense)

If you are just looking to change the gearing, check out denniskirk.com. Last I checked I think they had 530 sprokets it 4-5 different sizes for the FZ6.