Healthcare missions are changing and many countries, even those with poor health systems, are tighter about regulations in medical work.
Your massage therapy skills are unlikely to be recognized in many other countries. But they are valuable and could be used as a bonus if you find yourself participating in overseas work.
Your certificate as a medical assistant may not “transfer” into the health system of another country, but your experience in that arena could be helpful.
Most likely, I would see it as a way to assist mission clinics with the same kinds of things you might do now. This would include checking patients in, handling a bit of paperwork, taking vital signs and assisting with point-of-care laboratory testing. If you are comfortable with some basic things like urine pregnancy tests, urinalysis, finger-stick blood sugars and others this can be helpful to clinics without any lab personnel.
Some of the “bigger” medical mission agencies may find it difficult to place your skill set. But if you have personal connections to healthcare missions, perhaps smaller ones that focus on community-level clinics would be a place to start?