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.