Check out any of the thirty-eight Rear Hub/brake/chain components enlisted.
The most reasonably priced component used on the Kd125-a4 1978 Usa Canada Rear Hub/brake/chain is the Nut,8mm.
Some items from this specific schematic: Circlip,42mm, Panel Assy,rr Bk,buff, Rear Sprocket,47t, Axle,rear.