Or they can knock off Xinjiang, Mongolia, and possibly the Northeast (installing a puppet govt. there), and sent a punitive expedition to Beijing and back. The Soviets would probably fight until Mao runs or is overthrown, then call it good, impose the mother of unequal treaties on China, and get out.
Xinjiang maybe, Inner Mongolia/Northeast not a chance. The Soviet's will have the initial advantage, but once the massive manpower capability of a unified and stable China is called up... Yeah, highly unlikely the Soviet's will be able to wage a high intensity war against a 3-4 million PLA plus tens of millions of militia, think about the ammunition, fuel, and supplies the Soviets are going to need in a highly hostile territory.
Like I said, initial breakthrough, quickly bog down somewhere in Central Jilin, Hebei. After that, depends on whether the Soviets are willing to stomach the mother of all warfare, they likely claim victory and leave. If not, the Soviet's will be forced to transfer units from Europe and their entire logisitical network to the east. This will lead to unrest in the Warsaw Pact nations, severly strain the Soviet economy, and in the long run bankrupt the USSR.
Mao's China will now expirence an eurphoria that will make the Cultural Revoultion look like bland. You see, Mao wants something like this to happen. "Foreign Imperialists invading the ancestral land" will rally the nation like nothing else in the world. China will suffer, and suffer greatly in many areas especially in the regions near the frontlines, but in the end "People's Warfare" the strategy that won the Vietnam War, Afghanistan, and is the founding block of modern guerilla warfare counters a large conventional style warfare.
Only a short punitive war along the border be in the best interest of the USSR. Capture border towns and maybe push towards the provincial capitals, then withdrawl. A long protracted war will be devastating, a war of occupation will be disastrous for the USSR.