Would a carbon fiber mold that was formed to the inside of the oval openings be strong enough to hammer the lips into shape? If so, I would be interested in one or two if you make some.
I thought about routing an oval hole (forstner bit for the holes) into some hardwood and then cutting a radius around the hole. It could then be used as a form. That might be option#2.
I'm a little worried about tigging thin old metal so the option is to use one of those flare dies and cut the holes into the air cleaner than just tig in the upper and lower straight pieces. The other challenge if you make a whole piece to be tigged in is getting it bent to the 8.5" radius. Dealing with the existing hole is another challenge. I dunno....it all seems like a lot of work but some times I like a challenge not that I don't have enough to do right now.