As usual, all good information from @explorecaves, and I would add that if it were me, I would put a patch on the inside. Instead of duct tape, you can use Gorilla Tape and leave it in place. For me, I would use an overlapping piece of woven fiberglass cloth (not the stranded mat material). Either one is better than none.Start by applying some ABS plastic pipe glue, which you can find at just about any hardware or home improvement store, to the cracks on your saddlebag. Put tape on the inside of the bag, that way the glue gets locked in the crack and doesn’t drip out through to the interior. This glue will dry without heat, so if you leave it in place for a few minutes, it will be ready to use again.
If you want a more resilient repair, you can try something a little more ambitious with an ABS plastic dowel. Just grate this into a glass jar so that it fills up with the shavings, then add a methylene chloride product and let it melt down for about five or ten minutes. Once this is done, mix it with a thin wooden stick (chop sticks work great), and use the same stick to apply it to the crack. Once again, make sure you set something like duct tape, to prevent the mixture from leaking through. Let it sit for a couple hours in a well ventilated room, then use sandpaper to smooth out any areas that might have dried awkwardly.
Then you can paint the bags with paint of your choice.