You might try www.bikeboneyard.com
about getting a post '88 head. Not everything available is posted but the guy responds to email requests in a timely manor. As far as swapping heads from differant years it looks like heads from '83 to '00 are all interchangeable. I could be wrong but I checked out all my service manuals and all the head specs for '83 to '00 are the same. You might call Cometic gaskets up and ask what years thier XR600 head gasket fits. If it fits them all, hell try it.