[Willem Fick]

The first piece shown in your post is an aluminium part, so can't really be plated. Would respond well to being cleaned properly, either by scrubbing with an aluminium cleaner, by tumbling, or by using a very soft media to blast it with - I'm thinking dry ice... Painting it would not enhance the look, and as it sits in close proximity to your brakes, you will probably find them turning a darker shade of black again in no time.

The following two pieces shown are (ferrous) metal, and can be properly cleaned in a strong caustic soda solution. I guess you could paint them or have them galvanised (yellow if you want the cad plated look), but as they are part of the drivetrain not usually seen, I see no purpose to doing more than just a good cleaning job, which should do a more than adequate job of restoring the "metal look" you are after.