I d/sed a crf450x back in '08. Its been an awesome bike!
No issues whatsoever, no need to swap intake valves...the key her is to keep a clean air filter and make sure the things mounted correctly... its the dirt that gets the intakes. Tiny flat one ring piston keeps chugging along. Its a little weaker after 10000 miles but only by a tiny bit. Might swap into a TRX 2 ring version at rebuild, though it would hamper the 450's awesome fast spool up... Its got the full race Honda book mods done on it too, heaps of power.
No need for a 6 speed at all. I have to wonder about all the comments about the need for a wide ratio box...it comes with one! Its the R version that has the close ratio box! I can hit 100mph with 15/45 gears and its still low enough for most stuff. Technical single track s easier 14/45, and its still got a top end that can easily cruise at 70... I really fail to see the need for a 6th beyond just having one because the KTM do.
Added an oil cooler so it would hold more, though its really no biggie. Bigger tanks...I have several on up to the gigantic 7+ Acerbis. Add a stater, hand controls and lights and its good to go. In fact, I ripped all that stuff back off, unnecessary. Brake light switches and a rear tail with a license light are all thats required here in NC. Even dumped the soft bags too, backpack works fine. Trick is to travel with only the stuff ya need, take twice as much money and half as much stuff.
CRFX makes a great ds!