When I see a tree like that I tell people: Let's assume I can keep it alive...it will still be a deformed tree with weak form. It will also grow very slow for a few years as it rebounds from whatever caused the dieback. If you replace it, the new tree will look better, we'll get it started with a good young tree training schedule so it will have strong long term structure, and it will outgrow this one so that in 5-10 years whether you keep this or start with a new they'd be the same size.
Replace it.