Well it can't be a total loss cause since it is growing. I don't know if people graft magnolias but if it's the same plant top and bottom, then you're OK.

And since you mentioned that it is a shrub variety, then new growth from the bottom is good and you can cut the dead branches once you are satisfied that they are really dead.

My susan magnolia was only supposed to grow 8 ft. Cut off the dead. You have nothing to lose. And count your blessings. We have to keep pruning back our 12 footer!