I'd go and buy a decent bit of lite ply and use the originals as a template to cut your own non-warped fuselage side/top or whatever.
In the past, I've made fuselage formers by cross-graining balsa sheet and forming a ply construction, it's always worked well, but if the design originally used lite-ply, I'd stick with it, even if the kit manufacturer might have used the wrong grade of wood.

There are a couple of specialist balsa suppliers in the UK, luckily one near where I live in Essex. They will supply graded balsa and lite-ply to a cutting list; if you say what the balsa will be used for, invariably you get the right grade with matched pieces for each side. Surely there must be loads of them over there in the land of the free?