You might just be able to replace the outer plastic part. Looks very similar to this that Lowes sells:
https://www.lowes.com/pd/IMPERIAL-4-...nt-Cap/3203043
I installed a metal one like below because I didn't like the idea of plastic. But then you would have to replace the whole metal duct behind it.
If you leave it "as is" I would be worried that rodents might crawl in there.
I just used general outdoor caulk to seal around the edges. Seems to work fine.
You can use a 4" worm clamp to attach the ducting. You could most likely use what is all ready on there.