Assuming you can't get a gasket to seal it.#1 Weld up the low spots then machine it back to flat.#2 Weld a new plate to the old one and machine it down.#3 Remove the old plate and weld a new one in its place and machine it.#4 If you don't want to mess with it then just Replace it.I would do #1 or #3.
A candle burning on both ends is twice as bright but lasts half as long.