It looks like the zip cover is just jammed in the zip. If you pull the red flap out of the zip it should work fine if this is the problem.

that backpack can be kept functional w a new zipper

This looks like the zipper is jammed because it has ingested some of the backpack shell. Pull on the red material and back off the zip pull to free it. Be careful when re-zipping that it doesn't catch on the same piece of material; ?iron that bit to prevent it flapping into the way. Might work, worth a try!