I think keeping the Mongolian Empire alive for a bit longer might do the trick. This ATL Mongolian Empire would convert to Islam like OTL the Golden Horde, the Il-Khanate and Chagatai Khanate. The Yüan Dynasty was the only non Muslim successor state, so I think by keeping the Empire alive for a bit longer we may see the Yüan being Islamized.
It seems to me a bit to late, but then again when is a good time Islamize China?
The more I think about the more I think that Islam could never become a majority religion, but might be interesting as a big sect, (somewhat akin to India).
Optimal scenario:
Convert Mongols and the emperor.
(A very thorough government missionary program started).
The Mongols fall due to unpopularity and in their place a dynasty similar to the Mings takes over. Of course the missionary program ceases and is frowned upon, but 3-4 % of population remain muslim or influenced by muslim thought.
This very chinese version of Islam gains a bit more popularity in some areas (like???)
but then get a huge boost when the Manchu´s invade. (One result of the Mongols converting was that the Manchu leaders also became interested and converted as well).
Once again the emperor is Muslim. He is wise and makes sure not to piss anyone off. However once again the state is used to promote the emperor´s belief in a peaceful way.
In the 17th century the official imperial Koran is published. It has been influcenced by many things, buddhism, confucianism in particular but also Taoism and even Jesuits.
By the beginning of 19th century Muhammed the prophet is a popular figure (almost a demigod in some places) and Allah has lost a lot of his personality and is often thought of in terms of Taoism. Stories of Muhammed borrow a lot from stories about Buddha, although he retains many things from the original prophet.
In the wake of many catastrophes China deals with (such as losing a war to a western power, perhaps not Britain due to butterfly effect), Islam once again makes history for China. Some nationalists consider it a foreign negative influence, while many sects pop up in southern China. (A religious awakening).
By the end of the 20th century 5% of the Chinese population are muslims that strictly adhere to the laws of the prophet, many more Chinese pay respects to the prophet but worship other things as well (25%).
Maybe not a very exciting TL, but it sounds like the optimal to me.