If the top is truly dead, then you need to remove it. Dead wood provides a safe harbor for disease and insect problems to flourish so it should be removed. And you can do that at any time.

I'm not sure I'd attempt Katsura here in 5b because it's so early to leaf-out. Any late frost could zap it. Did it have any buds forming this spring before we got the late frosts?

I'm sorry about your tree. I'd cut off the dead wood, then leave the rest for at least a month or two to see if it puts out a second set of buds. I had a newly planted Kihachijo and a rabbit-decimated Mikawa yatsubusa that both took about a month to put out their second set this spring.

Thank you both... I will remove the dead wood and hope for the best!