If I had rust like that, I would clean it out thoroughly with abrasives and chemicals like Naval Jelly. Then I'd apply multiple coats of POR-15 or a similar moisture-cure coating to rebuild the areas structurally. Cured POR-15 will certainly support an air filter housing -- I've used it to restore battery trays. In fact, I used it to rebuild a floor rust-out just forward of the rear subframe mount on my '82 300 TD. After 9 years it's still quite solid, with the support of the nylon fabric I got from the POR-15 folks. It didn't stop rust from progressing elsewhere, of course. And yes, I can't vouch for its structural strength in a collision.