Since the orignal color was silver weathering won't be too hard. You can just use an exacto and scrape the black paint off a bit. Try and do it in places that get weathered, like the nose cone, wings, the sides of the ship, engine areas, and places around hatches and windows. I think the guns would look good on the belly of the ship somewhere near the back. I personally would make them look somewhat concealed, as if they were some sort of illegal modification.