If you're willing to go through the trouble and it's worth it to you, the battery manufacturer may replace your flashlight. I know I've read people do that for various products ruined by leaking batteries, but it might be too much hassle for a Mini-Mag.

Or you might try salvaging it. It may not turn on because of corrosion. The liquid inside the battery is alkaline, so you could try using an acidic solution to neutralize and clean it out. Some vinegar or lemon juice would be convenient acids, and an old toothbrush or skinny bottle brush will help. (To be safe, use gloves, eye protection, and good ventilation any time you're combining acids and bases together.) Then let it air dry thoroughly.

However, if the liquid got into the electronics part of the head, I'm not sure there's much hope. I've saved one or two flashlights from leaking batteries this way, but the corrosion was limited to the tailcap end of the tube.