Replace the mitral valve with either artificial or a transplant valve.
My father (at 85 years old - a very health 85....) had surgery to either repair or replace. Fortunately the surgeon was able to repair - a much simpler surgery. But the team was also prepared for a replacement. You are only going to perform open heart surgery on an 85-year-old person once.
My father wouldn't have lived more than a few months if he did not have the surgery. He came through with flying colors and at 95, his heart is fine.
"Time wounds all heels...." - Groucho Marx