First off go to a doctor today to get it on the record that you were sick today - they aren't legally allowed to backdate it and the BOS certificates are more detailed than normal ones.
Second still do the exam - that gives the BOS the option, if they grant you an illness/misadventure appeal, to judge whether you actually did do better than your assessment mark would give you - it gives you two chances at a better result rather than simply relying on your cohort.
I agree. If you can, try and get to a doctor today and get the certificate showing that you were ill today just in case.
I'd personally still try and go to the exam.
I mean, if your physically sick during the exam, it's probably going to be upheld. Of coarse, your still going to need a doctors cert. for tomorrow too, but a girl at my school was sick during the exams and out examiners were very kind and made sure she knew exactly what she needed to do afterwords because she didn't finish and what their role was.
I guess in the morning if your still unwell, call up your school and let them know, and let the examiners know before you go in in case you need to run outside and be sick.
I hope you feel better soon though! I'd hate to have something like this happen to me