Depending on what finish the fasteners 'should' have, for originality, you could de-rust them and simply satin clear coat (for natural steel look), paint them with 'cast blast' or similar paint for more of a factory natural look or buy one of Eastwood's (ot others)plating systems that offer a black phosphate which many underhood and underbody fasteners came in.
You'll need to let whatever paint finish you apply dry really well (several days) and then be very careful when re-installing not to remove the finish on the corners or edges.
