I may be getting the 250X and 450X problems mixed up but....from what I understand, you probably want to swap out the valves also. Stock valves don't like long, sustained high rpm. I've heard good things about aftermarket stainless steel valves.
Thumpertalk would be a good resource: http://www.thumpertalk.com/forum/80-crf450x/
"Correct spelling, correct punctuation, correct grammar. Hundreds of itsy-bitsy rules for itsy-bitsy people. No one could remember all that stuff and concentrate on what he was trying to write about. It was all table manners, not derived from any sense of kindness or decency or humanity" - Robert Pirsig, Zen and the Art of Motorcycle Maintenance