I'd be surprised if you didn't get in ED. Your ACT score is a bit low, but I don't think it'll hurt you all that much. You should have some solid experience to draw on to write a great essay (whether it be music or martial arts).
If you were rejected ED, I don't think it'd be due to test scores, but probably something lacking in the essay(s). So I wouldn't stress about scoring higher. If you can study for the ACT/SAT and score higher, great. If not, I don't think it'll destroy your chances by any means. Keep in mind that with admissions essays, you are trying to accomplish a specific task, not showing you can write well. Most people applying can write well and even if they can't, if they care about Cornell enough they will edit and edit their application essay so that at least that particular essay is well-written. You will need to demonstrate your "fit" and show/explain why you belong at Cornell; including both why you will be a valuable addition to the student body and what Cornell will do for you in your goals.