I agree with the recommendation of a soak in acetone. I'd follow the soak with a gentle once over with an acetone dipped cotton swab then placement back in a sealed 2x2. Much more than that and you run the risk of causing permanent damage, The token will never be "As New" again, but at least you can prevent further deterioration by removing surface contaminants.
Let us know what you do - and the results!
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