Without being able to have a close look (for small stress cracks etc) it's kinda hard to say, but here are some pointers;

First thing to do is remove it from the bike and work on a solid bench, preferably with a vice. Placing the flat part in the jaws will get it almost straight without over stressing it.

Or,

Depending on the condition, you could try pulling the lever straight, or using a hammer, or hammer and heat, or heat and pulling. Heating the lever up at the point you want to correct the bend will make it more difficult to work with (you'll need some really good gloves - welding gloves work well) but will put less stress on the steel

There is some adjustment at the piston (to the rear of the pivot), but it depends how much adjustment you need, it could be beyond the adjustment range, or it could be within it.

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