I really don't think it would be worth your time to touch it up. I have a ton of scratches on my cell phone, and it's just fine.
The reason I say it's not worth it is because you are going to have to keep re painting every now and then. Model paint isn't generally meant to be handled too much. In my experience, it rubs off way too easily when handled. It can stay on longer if you use primer, but not by too much longer.
Also, you would have to mask off the area that you don't want to paint, after you sand it down smooth, then prime it, then paint it. You'll have to do this each time the paint wears off.
Like I said, I really wouldn't bother with it, just because it would take too much time and effort. I just live with the scratches.
If you're still set on painting it, I would use a chrome spray paint, available wherever you can buy spray paint. I think I have used Krylon in the past. It dries more of a slver color than chrome. It is a tough paint, meant for use on car or truck bumpers. Use primer from the same manufacturer as well.