Just some guy
67 coupe, 69 Sportsroof, 86 hatchback
New repro parts are cheap and worth using. A patch isn't permanent. As "cut and replace" jobs go, replacing the front inner fender and battery tray is one of the easiest ones. It is kind of involved. You'll need to pull the fender, cut out the rotten piece, and weld in new pieces. If this is more than you want to get into, check out a catalog like NPD's. They have good illustrations of the parts you'll need (and prices). Take the car (and catalog if necessary) around to some shops and get some estimates of what the labor charge is to replace to parts versus a patch repair. Go with what you prefer or can afford. A 6 inch hole in the car is NOT good.