there's some real simple ways to cut out a patch and make a new patch. One way is to cut it out being mindful it comes out in one piece. Then file off the crap hanging off the edges, then you can lay it on top of new metal and trace it with a sharpie. The width of the tip of the sharpie makes up for the metal cut away with a cut off wheel. If you use a mini air saw to cut out the bad metal than the width of the tip of the sharpie may be too much and you'll be doing a lot of shaving to get it to fit. Anyhow, you then cut out your new patch and aim your cut off wheel on the side of the sharpie mark that leaves you MORE metal. Last, you bring the patch up to the hole and figure out where it needs to be shaved down and shave accordingly.
If the shape is pretty complex, measure how big your patch needs to be, lay it on top and hammer it into shape directly over the old rusted area(metal still intact), then lay it on top where it needs to be and scribe around it, and again cut on the side of the scribe that leaves you the most metal. The patch should be near perfect for the hole. If the complex shape is big then use self tapping screws and screw the patch in before hammering it into shape.