Yes, a good test score is helpful, but I do not think a good score in the ACT or SAT is going to make or break your application, given your circumstances. I would do as @skieurope suggests and contact those colleges’ international admissions officers, explaining your circumstances. I would probabaly let them know about the SAT subject test scores you took, and your grades, just to show that you aren’t wasting their time.
Remember that the overwhelming majority of applicants are going to have very high SAT or ACT scores. You have high scores on subject tests, high grades, and good extracurricular activities. If you are thinking of going to great expense and effort just to take an SAT or ACT, when the odds are very much against you (and everyone) I personally would not bother. Without great everything else, I honestly don’t think Harvard and the like will think “wow, she went to a lot of effort to take the ACT in another country. Admit.”
When tippy top colleges are considering which international students to admit, they are going to take the best of the best. If you are the best of the best, that’s what is going to get you in, not your ACT score. They are looking for the people who will make an impact, and your test scores don’t necessarily reflect that. Your recommendations and essays will be really important. The impact you’ve made in your school, community, church, etc…will be important.
You seem like a good candidate from the little we know about you. There’s a forum here on CC for international students. You can post questions there and ask others what they have done to get started with the process. Good luck to you.