If it's something major, for the Yammie 450 this would by far be the exception not the rule. I own a Honda and they are great bikes, particularly with the way they handle, but Yammies are by far the most reliable MX 4 strokes, and I repeat, BY FAR.
i'm glad you added the "MX 4-strokes" part of that sentence. i'll use my XR400 as an anvil to smash your CRF and your WR 450 and still ride it home.
of course... it's as heavy as an anvil, too
but that's alright, the weight keeps all 32 screaming h.p. in check. just for grins, i only know three fellows who have CRF450R's and two have had valve/head issues, hell, one is about an 8k$ bike.