I have a crawl-space door that is starting to look rather beat-up. I would rather repair the door rather then replace it as it has thick foam insulation on the other side.
What can I do to fix the bottom, and restore integrity.
Hard to tell... Is that bare wood at the bottom of the door part of the door or part of the door frame?
It looks like part of the door. Either way it's rotted or at the very least just swollen up from moisture. You need to replace the whole door. It's not worth the trouble to try and repair just the section for such a small inexpensive door. You can get sheets of foam to cut to size and glue on the back. You may even be able to peel the old foam off the back?
The frame doesn't look to be in great shape either. Might just need to be repainted or some of it may need to be replaced with new pressure treated lumber.
Since everything is so low to the ground you need to keep it maintained so water doesn't deteriorate it. May also want to consider revising the frame a little to keep the door a bit higher off the ground while you're at it. Provided you have a big enough opening.