[CarWiz]

First off, nickel doesn't yellow it greys or turns foggy. Could the parts be silver plated? I suspect the hardware is nickel plated brass and the years of polishing has worn the nickel off or very thin. You can plate over the brass and/or nickel after it is polished, cleaned and the old nickel "activated". To do it well, you'll need to remove the hardware though. The activation process involves soaking the part in a weakened solution of sulfuric acid (battery acid) for a couple of minutes. After soaking, rinse the part(s) in distilled water and plate immediately. Do not handle the parts with bare hands after the initial cleaning. Do not use rubber gloves. Touching the part with either will cause a bad plate.

Polish the part to the finish you want it to look like after plating. The plating will conform to that finish. Scratches will show as well as pits or flaking. You can do this by hand or with a buffing wheel and polishing compounds.

Clean/scrub the part with a heated degreaser to remove all buffing compounds. Follow with rinse then a cleaning with Acetone or Denatured Alcohol. Then rinse the part again with distilled water. (Tap water contains contaminates.)

From here, go to the acid bath for a soak. Rinse, then plate. Don't let the part dry out once it's activated.