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.