I've done a couple of bags ... even had a groan on here about inadequate bag zips. Get yourself onto 'that site' and look for long zips. I use the zips intended for boats or awnings etc with big teeth. I think zips are rated by the number of teeth per inch (?). I think I used either 8 or 10 teeth per inch. When doing the repair, I leave the old zip in place and stitch the new zip sides onto the outsides of the old zip. It takes time as I am no seamstress but it works. You will most likely have to use a zip a little longer than the old one unless you get a zip that you can cut to length.

Cheers, Phil