Maybe someone else knows for certain, but checking the diagrams at Partzilla, almost all of the part numbers are the same for the rear hub and brake. Same: coupling assy (
sprocket carrer), shock damper, axle, hub collar, chain, brake disk, and almost all of the brake caliper parts. The only difference I found was the brake caliper assy has a different number, but that could be because of the brake pads (?) >> the J model has 2 different pads listed but one of them was used on the CSR. I'd say it would work. guessing. ... ps: I don't know if the tire would rub the fender. I haven't done the circumference calculations.
www.partzilla.com/catalog/kawasaki/motorcycle/1981/kz1000-j1
www.partzilla.com/catalog/kawasaki/motor...e/1981/csr-kz1000-m1