Root stock is most likely just a generic green acer palmatum ...it will be upright and get big...and probably bushy with how it is coming out the sides.You of course need to cut the dead top off. It may or may not be a nice JM but likely ordinary and most likely outgrow it's current location although with alot of pruning it may be ok ... if it were mine i would move it ...but in reality it's sentimental meaning will be muted since it is not the tree you planted in the least...it will look nothing like it ... your decision of course ...David

If any of the original top is still alive I would cut off the new growth from the understock so the plant put all it energy into top growth. David

I'm from Illinois and with @ 50 Japanese maples, the same weather situation was horrible! I lost at least two trees and about a third of the rest are pruned to odd shapes! Good luck! I was told to use Superthrive from a japanese maple mail order nursery. It did seem to push leaves out faster on alot of the trees. Good luck, I feel your frustration. Charrube

Since the top is very dead, I decided to replace this tree. My favorite local nursery was having a 50% off sale on all JM over Memorial Day weekend. We picked out another Crimson Queen and have moved the old stump to a more suitable location in the side yard. The old one had a much larger trunk than the new one does, though the top is roughly the same size. In bonsai, trunk size is key, but in the nursery, to get the trunk size I wanted, I would have had to purchase a 6 foot tall specimen. And I would have done it, too, if the one I wanted had not had a sold ticket on it. Thanks, David, for the information. I see you have answered this same question in a dozen ways. You are very patient and helpful.

Brian