Looking at that last picture it occurs to me that, as well as repairing the crack and checking the closing plate, you should rebuild the lip where it has been ground down to restore some tensile strength to the arch. It looks as though the crack was caused by tension pulling the two sides of the wheel arch apart - and with the missing lip structure it didn't have enough strength to resist. The important thing in repairing the lip then is to include plenty of long fibers oriented around the perimeter, or lip, of the wheel arch. The long fibers will carry the load in tension. Chopped strand mat doesn't do a good job here, so woven cloth would be better. You can get woven tapes that may be a bit easier to work with than wide cloth.