Cross check frame mount holes with a tape measure.
Bolt it down upside down to the frame/deck of a car trailer. Get an 8' long strong steel lever and a couple of 4x4s and 2x4s as fulcrums and start to lift up any corner that is bent down too far. Out of square push it around with a port-a-power, heavy hoist come-a-longs and chain binders for holding it firm as you push. Can heat areas or cut a few minor reliefs with a sawzall to help things move, then reweld.
Straight and square, now weld the open seams and add a couple gussets where they will do some good.
Good Luck