Well, assuming you can remove the pieces to work on them individually, wash them in a solution of ammonia and water with a bit of alcohol in the mix. Dry thoroughly, the polish them with 0000 steel wool, and buff with a soft flannel cloth. They will gleam like new. Preserving that look can be accomplished several ways: Put a thin coat of good car wax on them, and buff out. You'll need to renew that eventually, but the shine should last several years in an indoor environment. Alternately, get a can of clear spray lacquer, and give the pieces several light coats of lacquer. Given how the rest of the clock looks, it should look like a million bucks with the restored brass parts!
Tony