Hi. John Steele at hi-speedmotorcycles.com in Chagrin Falls, Oh. is the man to talk to.
He has probably restored more XR's than anyone else. I found out about him in the early 90's when I was restoring mine. His website has everything you could want about xr's. Also Alan Girdler wrote a book...Harley Davidson XR750 which shows the history of the XR and has lots of pics and other technical info. His book can probably be found on ebay. Just search harley xr750. Hope this helps...Mike