I'm 90% there on buying a used '99 WR 400. ANy problem areas I should look for? I had a friend that bought a used CRF 450 and had quite a bit of work with the cylinder head. Without taking the thing apart, any good way of assessing how worn the engine is?

Id get a post 2003 450. Its much easier to start.

It should be fine, iv only heard good things about the 400 and 426. The crf's are prone to problems as they have titanium valves which cause problems and are changed out for SS ones.

Iv read about smelling the oil, apparently you should be able to tell if the engines bad.