Established crepe myrtles are IME rather hard to kill, they stump-sprout back... so I suspect that as long as you can coddle it along enough for new roots to grow, it should be back, from new growth at least (even if the existing branches die back to the ground).
If you haven't planted it yet yo might consider dusting the roots with a *light* amount of Rootone or other rooting hormone. Not lots, just a bit. And in a week or so give it a glug of properly-diluted transplant fertilizer.
Cutting back topgrowth is pretty controversial these days. I would not suggest doing it unless the plant is *wilting severely* and seems likely to die from overall drought stress. Just having leaves die and fall off is not the end of the world. Pruning would prevent the plant from being able to 'rescue' nutrients from those branches and leaves, and can potentially do more harm than good.
Good luck, have fun,
Pat