Did I hear someone calling my name....?
First thing to say is that these are some way from being the best fibreglass mouldings I've ever seen, but you've all worked that out for yourself, and I can see that it was made from poor quality glass mat, which makes me wonder about a few other things. Before doing anything though, does anyone know what shape the bottom of the boat is supposed to be, maybe it's meant to be concave like that? Assuming that it should be flat though, I'd say that any distortion is in the original mould, it wouldn't have occurred just from the moulding being pulled from the mould too soon while it was still a bit soft, though that wouldn't have helped. Can the hull be flexed into shape with just moderate pressure from your fingers, or is it too robust for that? If it can be flexed, it should be possible to straighten it, but heat alone isn't going to work. The way I'd do it is to cut a piece of 1.5 or even better 3mm ply to fit inside, chamfering all the edges carefully so it fits really well. Then, after sanding, dusting and cleaning the inside of the hull, cut a piece of glass mat to the same shape, and wet it out with resin inside the hull, I'd stick with polyester, though epoxy would also be OK. Then I'd support the outer edges of the hull with strips of 1.5mm ply, enough to force some slight convexity into the thing, you can always sand it flat later, and placing a pre-prepared length of straight and fairly hard wood on top of the ply, clamp the whole thing firmly to the edge of your workbench overnight. As an alternative, you could screw through a thinner length of wood inside the hull into something flat and solid on the outside, filling all the screwholes later, I've used this method to correct hogged hulls successfully in the past. If the hull moulding is too hefty to flex though, if you want it flat, you're going to have to fill in the concavity. The way I'd do it is to sand and clean the outside of the hull, and then epoxy a sheet of something like obechi the full length & width of the hull. Once again, clamp well overnight, and then plane and sand everything flat. Before doing this, I'd epoxy a few cross beams inside the hull to hold the shape.
Let us know how you get on.