Step 1: Paint the gun.
I used the cheap acrylic craft paints from Michael's (Americana or Folk Art) that you can find on sale for $0.39 each. For the main body of the gun (yellow and gray parts on the original) I used the Raw Sienna color. The recessed and raised details are Pure Gold. You will probably need two coats to get the right coverage. For the "metal" pieces I painted them Slate Grey first, then used my fingers to rub/buff on some Metallic Silver to get the right look.
Step 2: Dirty it up!
You may need a couple of coats to get the right look. Just let it dry and look at it again after a break to see what it still needs.
Step 3: Decorate it!
I chose these gold and crystal-esque buttons, but they had a buttonhole thing on the back that stuck out and prevented them from lying flat. Since they're plastic, I was able to use a hammer to smash the back a few times and then break off the sticky-out-y piece. Lay your button on a thick mat (I used my bath mat) before hammering!
I also felt that the bright gold color of the button wasn't quite appropriate for the antique look, so I colored over the edges with a black Sharpie.
Once the button backs were flattened and the front was "antiqued," I used hot glue to attach them over the logo.
I also found a couple other buttons to put in the grooves on either side to dress the gun up some more. I chose a wing for one side and a wood-looking button with a tiny crest for the other side. I attached them with more hot glue.
On the top of the gun, just for a little more visual interest, I attached the handle of a plastic paintbrush that I painted gold and a brass capnut.