In the times of modern technology, it should not be impossible for you to use "ä" or "ö", when required. There are various ways how you can do it. However, if it is impossible for some reason, I would prefer "a" and "o", like international media has done with the Finnish F1 drivers' surnames Häkkinen and Räikkönen (Hakkinen and Raikkonen). The umlauts look AWFUL in Finnish words. Most often Finns will be able to "correct" the words not written with /ä/ or /ö/ in their minds to the right spelling.