You could try pollarding or coppicing it in the late winter.
During the winter all the sap drops to the roots for storage. If you cut it all off a few inches from the ground you’ll get rid of the rotten bits and in the spring the tree will just start over, but with a bunch of root.
There’s some resources you can google if you use coppicing as a keyword.
This is a layperson’s advice, so grain of salt and all that. An arborist would probably know better.
