Sorry to hear about your lower back problem! First off, do you know what causes your lordosis? If you have overly-tight lower back muscles that are causing that exaggerated curve, you’ll want to make sure you get in lots of lower back stretching and massages if possible. This will make it easier for the antagonist muscles (in this case your abs) to work out if they have less resistance to put up with.
Now as far as exercises: try lying on the floor, legs straight and arms at your sides. Now bend one knee and put that foot flat on the floor, so that your leg is basically making a triangle with the floor. Bring your lower back to the floor (think about pulling your belly button down into the floor) and hold for a few seconds then release. Repeat several times. Then switch legs. It won’t feel like you’re doing much at first, but you’ll feel it if you do it long enough!
Another suggestion is to make gravity less of a problem. When you feel ready to try crunches again, use an exercise ball. This way, instead of laying flat on your back you are already sitting up at an angle and have less gravity to wrestle with. Plus, the ball provides great back support!!
HTH!