The first thing I do is check the diagrams & cross-reference at Partzilla, then search the part numbers on eebbaayy. sometimes just to see what a part is supposed to look like. NOS pics can be handy. Z1Enterprises dot com doesn't list a KS125, but they still may have some stuff for your motorcycle. Z1Ent is where I get most of my maintenance items as well as other stuff. Their pricing is as good or better than other places. PartsNMore is another site I use & are the only place I know of that have reasonably priced muffler connection gaskets for my KZ650.
Kawasaki dot com also has diagrams, but Partzilla is easier to use.
1981 KZ650-CSR. Competition, Sports & Road.