cut it out as a square and weld in some thin plate for the skin then weld in the thicker plate to the inside of the wheel arch.
them things are only spot welded on! if you seam weld it, it would be much stronger that it was originally
edit.. actually forget that.. i did not realise that it had holes through it! i thought it was shiny metal!
hmm you could make a new plate up by welding a nut to a thick-ish bit of plate with a hole drilled through it..