With steel wool, but you won't get it to look the same as the random effect on the original. You will have to rub the whole surface with the wool.
If the scratches are deep. you are onto a non-starter.
You can get special cloth wheels to use on a bench grinder. The important part is the polish. See if you can get the end of a stick from a local electroplating firm, the stuff they use is great. Hold the polish stick against the spinning wheel before buffing. Be careful to stay away from the back edge of the item you are polishing on the downstroke as it will kick out of your hands.