I would cut the dead part away a little back to live cambium. Even taper the cut a little further on the side away from the new trunk so the trunk flows a bit more smoothly.
The new secondary branching all needs to be either wired into position or pruned to grow into branch structure.
The apex probably need to be a bit shorter now too but maybe that can wait until the wound has healed.
Ground growing is good for increasing trunk thickness and healing cuts but just be aware that branches can get too thick when ground growing. You may need to start branches all over after it has grown some.