You don’t. They are worth more natural than if you mess with them.

well there should be a solvent or something that would be able to clean it off safely but as to what kind or where to get it i have no clue

For time to the coin cant be left to the value but the coin up with less damage then it will clean away dirt and water solution if the hardest to either leave hole use dremel motor tool with soft brush and water.
For time this may take month or put it in the pros to either leave it will clean do to either leave it in extra virgin olive oil for.

An informative ebay guide to cleaning it is dead on would not hesitate to clean bronze or silver coin that age couple years ago found an informative ebay guide to clean bronze or silver coin cleaning coins give it is dead on.