It looks like some of the plating has come off on the decorative bits.
How much do you want to take off? Muv has a great video about doing a gentle cleaning.
It looks like decals have already been partly removed. You can go with harsh chemical cleaners if you aren't worried about paint, decals or your health. I have a Singer 15 machine looks like someone used a belt sander to take off a good deal of paint. It will have to go way smooth to repaint. It is a lot of work.
In my opinion, I would clean up the filthy stuff and use it as is - I bet it will make a good stitch - consider the cosmetics to be war wounds or merit badges.