Polystyrene melts to a liquid at about 460 Degrees F, begins to transition (soften) at about 203F.
Water boils at 212F, so hot water will begin to soften the plastic.
If you are out about 1/2" at either end it sounds like a large project. Sometimes because of the casting you may find some sections are more curved than others, so if that is your case, it is best to find those sections and focus on them.
First rule of reshaping warped plastic:
Do not try to reshape the item all in the first try. You may have to go through a number of evolutions to get it right, so make up your mind from the outset that you can commit to at least a few dozen evolutions.
If using near boiling water set the item so that you can pour the water over the piece without scalding yourself and allowing for two 'grabs' that will be cool enough to touch.
If you've never done this before be patient and start out with water at about the 190F range.
Pour the water over the pieces for a few seconds and use the grabs to test rebending with the opposite curve, try to match the degree of warp in the opposite direction to see how the bend feels and reacts.
Another approach is to fill a pan with the hot water, bend the piece and dip the part you want reshaped into the water.
This is a learning process and every re-warp is unique, so you will have to test the process a few times.
Again, be patient. Some styrenes are brittle even when warmed they will literally explode into pieces if you bend them too far too fast.
As your confidence for the process builds try hotter water and slightly more aggressive bends.
Here's the trick: once the piece is heated and you have it in the new rebend poition, hold it there until the temperature drops to the point where it is below about 100F: you can feel it isn't too hot to handle.
You should notice that with each evolution the piece begins to approach the desired shape. You will get to a point where it may not be perfect but can be taped and glued, or you can press on to perfection.
Patience and safety are the key.
Good luck with your project,