The Sassinid Empire reforms and gets rid of major political instabilities in the 600's, and it manages to defeat the Arabs during their conquests. Later, it begins to spread Zoroastrianism into the north, eventually reaching the Eurasian Steppe, where the conversion is left to nomadic tribes.
have the turks conquer iran earlier and thus converting, this would not spread the religion to the Mongolian steppe or north but its the closest unless zoroastrinisim is heavy reformed by the turks