How to do sit-ups or crunches with a disability?

There are ab/core exercises that can be done sitting and standing. 
Here is a link to some seated ab exercises: https://booknicksanto.com/5-seated-abs-exercises-to-try-at-work/
Here are some ab exercises for those in a wheelchair https://livehealthy.chron.com/good-waist-stomach-exercises-wheelchair-bound-7349.html
If your balance allows it (and I realize it may not) here are some standing ab exercises - you may be able to modify some to fit y our abilities http://www.turnstep.com/Patterns/2006/March/Mar09.1.html

Can you bend your knees over a couch (up it, feet on the seat) back on the floor? Then rise your torso up but keep your legs resting on the couch. There are other moves you can do in pilates that work your abs while laying on the floor but have your feet in the air, if you can do that. Search on youtube for ab pilates.

You might consider doing abdominal lifts instead of crunches. This is a Yoga exercise where you exhale your breath and then use your abdominal muscles to pull your stomach area in and out in repetitions. I admire your tenacity to exercise despite your disability. I shows self-discipline.