you compare your kx250 and kx500 to some guy's kx450f? the blown head gasket could be an mftr error, and i used to have a 98 kx250 and after 8 years on the same piston (who knows how many hours) the thing was still in amazing condition. the crankcase seal went out and all the oil went with it and destroyed the engine, but all of the mechanics that looked at it said it was a freak accident worthy of happening to any bike. but still she ran rock solid up until then.
I am sure the blown head gasket was a manufacturer error. However, doesn't that seem ridiculous for a new bike to have a problem off the show room floor? When I spend $7000 on a new bike, it better be perfect and all my new Honda's and Yamaha's have been. Weird!