If you warm the piece, the metal can be made more malleable, and you can carefully bend it to shape. You could try a hand-held hair dryer, or even placing the piece in a warm oven (lowest setting possible). Then use a clamp to true it up to the other half.
I run across the same issues with some of the kits I build, like old Imrie-Risleys or Tradition kits. I have a French dragoon circa 1745 whose horse has a similar warp.
Now, in the "Old Days", someone might have said, too, to place the part on a flat surface and gently tap it with a piece of 2x4, but I wouldn't go that far. That can work for metal mold halves that get warped out of true, though.