What about hand sewing? The holes are already there, if you used heavy duty thread, you might be able to fix it going through the existing holes.
I've done some hand-sewing, but there's a lot wear and tear on a zipper, and if the zipper fabric eventually rips it will be a much bigger job to replace the zipper than just to re-stitch it back into place. I suppose if your hand-stitched it and left it unzipped it would be okay.
But it would probably be best to have it repaired professionally, if that is an option. Just my opinion.