If it really is a coin of Napoleon III, it is bronze. It is so far gone that I'm sure a cleaning will not hurt the monetary value (if any is still left)... Thoughts? Numista link for reference
Perhaps leaving in a dish of distilled (definitely not tap water). If that doesn't work perhaps a dollop of Brasso and wire wool then retool the design back on!
My opinion, there's not much you can do to help that coin. But if I was going to do something, I'd use Verdi-Care and let it go at that.