The intakes are prone to "cup", if you keep the motor stock, you'll have to adjust the valves less frequently. I have a modified 97 KLX 300 and in the first 2 years, my valves did not need any adjusting. I put on about 20 rides @ 50 miles per ride per year.
In year 3, things changed, I had to adjust the valves for the first time by putting in new shims on both intakes. This job is not to hard, just need to use a little common scense and take your time, work the learning curve and it's done.
In year 4, had to do it twice, and would have had to do it a 3rd time, but ran of specs, so I just recently pulled the cylinder head and you can definitely see the intakes worn.
Never had a problem with the exhaust valves, nor have a heard of anyone else having a problem. If you can't get a manual, checking the valves is extremely easy. Remove (Gas, or Petrol tank), then pull the valve cover off (3) bolts, use feeler gauge, record your measurements, put grease on rubber valve cover gasket (inside w/ the nurls) and reinstall.
I suggest going to
http://www.buykawasaki.com, you'll find the valve adjustment specs and all the parts schematics, for the KLX 300.
Hope this was of some help !!!
MC Out !!!