It appears that the metal is in good shape (these appear to be fractures, not rot) A good welder should be able to use small patch pieces and make this repair, avoiding major surgery. The olde rule is replace to the factory lines when possible or use the smallest section of replacement panel as you can. AMD came along as sort of modified some of that rule but it still mostly holds true. Biggest trick is clean, clean, clean. People try to weld and leave crap next to the run and get all kinds of contamination and porosity or they wont pay a professional (when needed) and keep pouring dog **** on top of crap. Then they throw a **** load of paint or seam sealer at it and act all proud.