Very risky because any type of cleaning that will remove the crud build-up will also probably remove more of the surface - those pink spots, and even if you get it "clean" too much pink will make it less appealing than it is now. I personally would be very hesitant to try anything. In fact, I can almost say 100% if it were mine based on that pic I'd be leaving as-is.
I normally do electrolysis on the buttons I find in my yard but after the last one fell apart I said no more, this one is the oldest one by far Ive found at my house. That why I was asking.