Depending on the extent of your rust. If there is a lot of rust this wouldn't be the best plan. But for a small hole this is the best choice. I had to repair my side cowl vent do to rust/vibration cracks.
First remove the vent door.
Then look at it. Think about what is going to be the best way to work on it. Cutting it out fabricating it and putting it back in. IF you don't do this it may take a lot longer to get it looking good. Particularly fabricating the new piece. If the bends arn't bad you may just be able to bend it with the vise but if you got a nasty complex bend you may have to cut several pieces.
Mark what to cut out. Then cut it. If the angle grinder or die grinder is to big use a rotary-tool (dremel) They work wonders on small body repair. You'll burn up quite a few cutting wheels but their cheap.
After you get it cut out. Make the piece to fit right in the spot you cut out. Most people like to overlap everything. Which is a lot easier but with micro body work 2" square or less it wont look right if you do this.
Once you get all the bends just right, weld it in. It will be hard to do a butt weld on this thin steel but it will work. You may burn through a lot and then have to weld holes shut. It's usually unavoidable. The way I take care of the crappy welds is grind it down. Then weld it over again. Welds better the second time.
Once you get to grinding first or second time a tradition angle grinder often is to big. Once again go to to rotary tool. The rotary tool takes a while, but it will do a good job.
After that's done a little bit of bondo and hand sanding will leave that cowl looking untouched. Particularly after you reinstall the door.

1947farmer

1947 2-Ton Chevy
Story in the Stovebolt Gallery
More pix in Photobucket
1977 3/4-ton
1985 2.5-ton
2- 1947 Allis WC
1- 1949 Allis allcrop
1- 1955 Allis WD-45
1- 1976 Allis 7040

"If it ain't broke you ain't trying"