The easiest way to check is to look at the steering stems of each bike.. If the diameter on one bike is different, forget it.. If they are the same, then you need to look at steering stem legnth and setup to make sure its the same or close.. Then you will have to think about brake setup.. Are they both disk brakes or is the xr drum? If they are both drum or both disk, it might work.. If they are different, it won't work.. Then there is wheel setup.. Youll probablly have to use the cr80 wheel, axle, forks, tripleclamps, and brakes from the xr the steering stem might be required. And lastly youll have to figure out what fender to use, but since your useing the cr80 clamps probablly the cr's fender will work best....
Good luck!