YZ400 counter balance mod

I have just bought this bike and noticed some noises I thought were a little out of the ordinary. I pulled the valve cover off to check my valve clearance and it seemed to be fine but while I was turning the engine over with my breaker bar I heard a clicking or light clunking noise. I then removed the flywheel and found the key way pretty loose. I put the nut back on and tried turning it again and the noise went away like the key way was the problem. I have read about a counter balance key modification on other bikes but was it a problem on this bike as well? If so where do I look to find the kit? I plan on replacing the Chain while I am in there just to be safe. Thank you for any input.

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