oil leak

I own a 98 ZX6R. When the bike was new there was oil leaking from the
cover on top of the front sprocket. It was fixed under warranty. Now, 4 years later the problem started again. I do not want to pay through the nose to fix it. Anyone have any idea what this is and how difficult it is to fix. I took it apart yesterday, and did not see any oil inside the cover. All I saw was grease and dirt from the chain.