The XR3 is most likely MFI, it was only the later ones that were EFI. You should be able to transfer the head. No wiring etc needs to be touched, you may if it's EFI need to drill a hole in the block for the CPS at the most.

In any case how do you know that the top end isn't at fault? Valve stem seals are a weak point on the CVH thus cause oil to be burnt, that is the usual suspect!

Martin