Err, aside from the insane distance? How exactly are they going to break the Tang dynasty? Mind that the Ummayyad may have lasted longer but their downfall (Abbasid Revolution) was around the same time as the An Lushan Rebellion. They're not going to have an easy time managing all of Europe, with how they treated non-Arabs (Great Berber Revolt, for example). Add Central Asia to the mess and it's a matter of time for the whole establishment to crumble apart at the slightest breeze, with an attack on China being a storm in that case. Aside from souring the Chinese opinion of Muslims for a while, the Ummayyad (or any Muslim power) would have to actually be able to get enough troops from their heartland over all of Central Asia, over the mountains guarding China's heartland from the west, over the Yellow River whose floods kill millions each time, which is around 2600 miles. It be would fair to say such a feat would take at least a year to travel, several more just to plan with an army large enough to subdue China? Say they plan it perfectly, weather and supplies stay passable (somehow), don't get raided silly by the Turkic nomads, and now they have to fight the Chinese. Say they conquer it without rebellions in every non-Arab part of the empire. Say they get the majority to convert.
Have to assume quite a bit, honestly. Anyways, China's not going to get Islamized so much as Islam (in China and the rest of the sinosphere that accepts Islam) gets Sinicized like Buddhism did. In the end, China conquers its conquerors (Manchu, Mongols assimilated into Chinese culture). Arabic's not going to supplant Chinese script, perhaps more influences but it'd take centuries to replace Chinese script (Pahlavi script endured centuries after the Muslim conquest, I believe) and assuming a nation whose administrative capital lies thousands of miles away from China can actually hold all of it for more than a couple years is very, very optimistic. If the Muslims move their capital to China, well, then we have the Mongol situation a couple centuries ahead of schedule and it'll fall apart anyways due to revolts, ethnic and religious issues, succession, political intrigue, who knows why but continuous empires with large populations on opposite ends tend not last for long.
Spread by trade seems a bit more...plausible, I think. But, then again, the Tang weren't exactly open to outside faiths (Manicheanism, Buddhism, Christianity, Zoroastrianism). Keep Islam low profile for a while, have it avoid the persecution of other religions, expand trade with the Islamic SE Asia, you can get a pretty decent Muslim population in China without a drop of blood and without a legacy that might get it wiped out in the future, much like in SE Asia.