try to see if the crank assembly are the same for both XR\XL.. if yes, there should be no problem to mount the XR flywheel on the XL crank also check if the stators XR\XL have the same mounting holes pattern.
i guess both stators have 2 coils\circuits: 1 is for ignition and 1 for lights and other stuff; this one, on the XL goes to battery charging circuit, on the XR there is only a capacitor instead of the battery
the headlight (if i remember well) should be AC current in both series XR\XL, while the minor electrical stuff are DC current (at least on XL)
probably you can drive the XR wiring with the Xl alternator (stator+ magneto flywheel assembly), but must study well how the whole thing works.
note also that the ignition black box should be different (more advanced \ different curve) on XR
also: different marketed XR s came with different electricals: for example australian market XR have blinkers, italian market XR600 had a bigger headlight, with more coils in the stator to drive it (it was road legal, but no blinkers fitted..)