Exhaust Valve dropping

[mtbdrew]

My son bought a 2002 GZ250 that according to odometer has 4600 miles on it. He added another 70 mile and the bike stopped running as he drove down the highway. Found the exhaust valves had broken off third the way up and destroyed the head assy and piston. We ordered a replace jug and head assy from China and got the bike up and running again but 32 miles later the bike stopped on the highway again. Took the cover off the head assy and found that one of the exhaust valves had pulled through the cotter and dropped getting hit my the piston and bent. The other valve was well on its way out of the cotters too.
Could these have just been bad cotters? Or is the bike running too lean get too hot and causing the valves to drop?
Everything is stock on the bike except he did replace the air filter with one that doesn't have the scoop attachment and is supposed to be "high flow". could this be causing the above problem by making the bike run too hot leading to these odd exhaust valve issues?

Thanks
Andrew