If you remove everything by topping the tree you could graft onto the trunk. It will have a well established root system. Grafting plums is not really hard if you've never done it.

Black knot is what discourage a lot of people from growing plums esp.in humid area. I would make sure I burn those wood or carefully put them in trash bags. Try not to drop too many spores on the ground.

If you want to grow plums, will you be willing to spray for black knot.

There is no saving a plum well-infected with black knot. And it is systemic to the tree so don't bother attempting to graft either.......the grafts will eventually get it as well.

Most issues with black knot are vectored from wild plums in the area so very difficult to control without the removal of all the host plants. The time to spray for black knot is before you see any signs - the fungicides used are only preventatives. They can't 'cure' an existing problem.