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Yes, you can do sit ups and there are many other elements you can do that hit the abs.
Hollow rocks are one.
Come up to 90 degrees when doing sit ups
Have you done anchored sit ups?
Have you done sit ups with the bottoms of your feet touching each other?
This takes the hip flexor out of the equation.
Be smart about what you are doing and pay attention to your positioning throughout the movement and over whatever number of reps you are completing. Quality over quantity.