valves?? for a 250x

hey i lost compression and my quad wont start no matter what and someone i know(good with machines) said that one of my valves might be bad. if it is my valve should i replace all of them and get performance valves? what brand? any other things that could go wrong that would make me lose compression before i start replacing stuff?
thanks shaun

aol AIM s/n=nismo360

tha valves for my 91 250x were 12 bucks a piece. the valve seals were shot also. they were 4 bucks a piece. she's runnin fine now, i would replace em and not waste extra money on it. put the savings into a faster stock bike like the 400!