An American wouldn't have being able to invest in the Chinese market in 1970, Mao was still alive and foreign investment were almost non-existent before laws allowing it were passed in 1979.

In case this counts as a what-if question, the real question is what has been the investment environment for average American investors in China in the 70' through 90's?

An "average' American wouldn't be investing in China in this period, precisely because the legal status of FDI was ambiguous and subjected to heavy restrictions. An average person invest in a portfolio in the US would be doing so through a stock market index fund or the like, but stock exchange didn't exist in the PRC until 1990, and even then the market cap of the SE was very low as percentage of GDP thereafter.

The lack of a stock exchange as a capitals market meant investment basically precluded western individuals, they were limited to "joint-ventures" with Chinese companies, which had to be done by western corporations. The point when a diversified portfolio was possible was probably in the 90s.

The people who were most successful at investing in China during the period were overseas Chinese, Hong Kongers, and Taiwanese investors. Because of their knowledge of language, culture and customs allowed them to more easily navigate the complex environment governing FDI of this time period.

To put things in perspective until 1995 foreigners were not legally allowed to own or use RMB, Chinese currency. You had to use foreign exchange certificates, although there was a black market for foreign currency. In the 1970's there were not private companies to invest in.

The only securities related to China that you could invest in are pre 1949 defaulted bonds. In the 35+ years since the 1970s only a selected handful of these bonds were paid out to British holders. If Taiwan reunified China has agreed to repay those bonds but so far they have been terrible investments.