2007 kx250f problem

[Jeremy2566]

I am about 5'11 180lbs looking to get my first bike for a woods and trail bike.I am looking at a 2007 KX250f bike and the owner says it runs fine, just shimmed the valves, could use a new back tire, has the 2007 kx250f known problem of leaking oil out of the overflow if you start it up and let it idle, but once you start riding it doesn't leak at all. How can that problem be fixed?