Indeed, thats a good question. Perhaps ore?
The infrastructure for that didn't really exist--one can't export ore without already having railways and mines to efficiently move it to harbor.
I suppose Mao's China can try the route of exporting crops, whether rice or wheat, to places like India or south-east Asia, but it's in a rather more difficult position than Stalin's Russia was. Parts of the West, after a while,
did open up to the USSR--the hope was that the introduction of capitalism would, by the 1930s, liberalize Russia from within. Stalin took advantage of that to export wheat after he came to power. But American, and by extension a lot of European, foreign policy in the 1950s and 1960s was explicitely aimed at isolating Red China from the most obvious markets, Japan and SE Asia. If Mao wants to keep Soviet support, he won't try to liberalize first in order to allow his PRC access to these markets.
This means that we need to go back to the mid-1940s, at least. We can't have Nationalist China be as completely defeated as it was IOTL--we need a coalition government of Mao's communists and the Nationalists, and for the challenge of this thread, Mao must be dominant at some point for an extended period of time. The coalition-nature of the government will grant Mao's regime some legitimacy in the eyes of the west, and that means open markets for agricultural export to finance these industrialization measures. Stalin would be open to such a thing--he supported the Nationalists until at least 1948 anyway. This scenario, however, requires a less 'ideologically-pure' Mao, one more willing to see reason and not try to get all his revolution at once.