Enamel paint from a hobby or craft store like Michael's works great if you can't get an exact match from your LBS. Small bottles are less than three bucks and offer plenty of paint for touching up scratches. They will also have some cheap brushes in a multi pack for a couple bucks as well. You should get rid of any loose paint along the scratch and then take a tiny bit of sandpaper to remove any oxidation on the metal. If it's steel and there's rust, definitely sand it away. Aluminum oxidation isn't as much of a problem but I would still sand it. Just wrap a bit of sandpaper around a nail and you can minimize the area you are sanding. Wipe away any loose dust from sanding and apply your paint. It's best to do a number of thin coats than one thick and heavy coat. You can add clear coat if you want but often it's not necessary with a decent gloss enamel.
Good luck.