Most Bavarias do not have hull anodes, only the one on the saildrive and possibly the propeller so this seems like something that has been added. If it is round then simply replace it with another round button one. You will need magnesium if it is in fresh water. Don't know where you are so diffficult to suggest a source, but anode manufacturers such as MG Duff or Zincsmart will have them in their range.