I don't think you can do it again-they'll just use your assessment marks.
So-if you are absolutely physically unable to sit the exam, then you have to hand in a docters certificate and hope for the best. The problem is-if they on't accept it, then you're screwed. So-If you can manage it, then still get a docters certificate, but sit the exam. Tell the supervisors beforehand that you are applying for special consideration (they will ask everyone before the exam starts), and they will kep an eye on you during the exam and write a report on how much it held back your performance. Then they take it from there to figure out a final mark.