It is my understanding that it will be a direct fit.
There is a bit of an electrical concern. To run the XR600 electrics you will need to swap out the XR650L stator with an XR600 stator. The 600 stator has the magneto winding for the CDI where the L stator is only wound for 12 volts.
Mechanically, yes, it will fit. Like cleonard said, the 600R ignition is configured to run on its own dedicated high voltage AC stator windings. The 650L's ignition uses regulated 12v DC from the main charging supply. Assuming you still have it, just put the 600 stator in the 650L engine.