If you are willing to count Xinjiang as China to fulfill this condition, your best bet would be any circumstance where Islam does not dominate the Middle East. Central Asia became Muslim in large part due to trade influences, and before then the area was a massive melting pot of religions, including the Zoroastrians. Allow Zoroastrianism to reform itself in Persia into something with a missionary or more mystic tradition, and you could see a stronger presence of the "fire-worshipers," as the Chinese sources called Zoroastrians at the time. It'd be pretty difficult make many inroads into China itself, though if somehow Zoroastrians tried to present themselves as Buddhist, or some sort of odd Buddhist tradition you might make some headway. I think most likely is that you might get certain concepts or some practices that derive from Zoroastrianism, rather than full blown followers themselves as being adopted into the Chinese Folk Religion, a sort of Crypto-Zoroastrianism. Otherwise, Zoroastrians will inevitably be non-Han ethnic groups.
Could have the Chinese equivalent of Parsi communities in Chinese coastal cities as traders and (especially) astrologers, both of which Persians were already well known for. Have a longer lasting Zoroastrian Persia develop a stronger maritime tradition, and you could feasibly do that.
I am skeptical of this opinion, it approaches the situation of Central Asia with the mindset of the period of the Kushanshah and the periods very directly following. The scenario at the rise of Islam is different from this period. As the opinion goes, the Caliphate invaded much of the region that we call Central Asia and inserted itself as a dynamic power broker, at the gradual expense of the Tibetans, Celestial Turks/successors and the Tang Anxi protectorate. Islam spread to the Turkic peoples and the region of Central Asia thus not through trade, but the insertion of a dominant military force and through the fist of the conqueror. In a world without Islam, the situation in Central Asia is chaotic and filled with competitors, only one of whom practicing Zoroastrianism, that being the Sassanid Empire. The Sassanids, though effective in defending itself tot he east, since the decimation of the Hepthalites, were certainly unlikely to make any leaps into Tocharia, even retaking Ferghana is a difficult proposition or asserting their power over Kwarezm, without paying tribute to the Celestial Turks.
In short, the way in which Islam made itself powerful in the Central Asian region, is something, that I believe cannot be repeated by the Sassanids and even a renewed Persian power, will need to find a period of quiet and peace in the western front with Byzantium, before it can do as the Caliphates did and enforce itself upon the Turkic states and stop the arrival of the Anxi Protectorate or the Tibetan Empire (already making itself prominent in Kashmir and the Kabul Shahi magnates.
Now respecting the means by which Islam expanded, it is important to note the spread of Buddhism/Manichaeism/Nestorianism to China. Namely, this spread occurred via the great expanse ruled by the Kushanshah and their openness toward transfer between Hindustan and the nearby Tang Dynasty and the intermediate Tocharian lands. By the time that we have coherent Zoroastrian thoughts and ideology, the situation in the region is changed, the Han Dynasty no longer exists and the region of Tocharia is unstable, the great Kushanshah are also no more and the Silk Road is in shambles. The peaceful means by which in times of trade that the Buddhism spread or other Dharmic ideas reached China, are no longer possible, until an hegemony is reached in the region that also maintains fair relations with China and thus a free movement of religions can sneak into China as Buddhism and other Dharmic related ideas began to reach China as a result of the Kushanshah and Tocharian connection.
