The rear wheel will be the issue. The four stud hub pattern was also used on the airhead monolever bikes, but I recall reading somewhere that when fitted the rear wheel has a different brake drum depth/offset.
I believe that some of the K series (brick) models had the same four stud hub pattern but I haven't heard of anyone fitting them to a paralaver (R100R/GS).
If you ask nicely I'm sure that there will be someone who lives near you who will have a bike with the correct rear hub fitting, who will let you try their wheel on your bike for size/fitment.