Yes I have tried one. Just as OZLinc said there are a couple of small difference's that prevent it from being used. I wont charge you for my "knowledge" this time. :tup: Next time bring a roll of nickels
The XR head has dual exhaust ports I think which may or may not be better, only testing would decide on which would be better. I feel that Honda would not have changed a whole lot when designing the XX engine from the XR so its gonna have to take someone trying it and see whats up. The 660 Raptor guys can put the 700 cylinder and cylinder head on thier cases with some work because someone sat down and figured out what it would take. The XX guys would have to have the same thing happen to see.
The XR head could be made to fit but would not be rational to do so. The XR does have two exhaust ports as Bo mentioned. The XX has one but for a reason. The XR weighs in at about 250 and the XX at 500. The single exhaust port gives the XX allot of needed torque. I honestly don't think the little bit of top end HP were losing due to it is worth worrying about.
If someone were looking to build an all out race bike the XX is not the quad to do so with considering you cant really stretch the rear. The XX head has potential to meet any power requirements needed, Bigger can be better but I believe in the case of the XX it is not.