The cylinders, heads and crankcase were natural, the carburetor covers, air cleaner housing and the outer fin on each head were polished, but still unpainted. The right side clutch/carburetor housing and left side carburetor & sprocket housings were painted with a silver metallic paint. Someone else may know of an exact match, but I've always used a Dupli-Color GM silver for 80's vans that seems to be a close match.
Someone else may have more exact info, including how to keep the natural aluminum cylinders and heads looking good over time. If you bead blast them to get a new clean look, the surface will eventually deteriorate and lose the "new" look.