I would start very slowly with the crunches. It will get better as your ab muscles gain strength. Do not push through any pain. Pain means, back off, fix your form and go more slowly.
So, for crunches you may do well to put your hands on your legs and use them for support rather than putting all of the pressure on just your ab muscles. You can also wrap a towel around your waist and pull with that if it works better for you.
These modifications are specifically for diastasis recti issues, but would work for cesarean issues as well.