You need to do a few things.

1. Deng Xiaoping must be the leader of the Reformist faction.
2. Do not actively eliminate Reformist politician influence. This means do not compromise with the Gang of Four or attempt to eliminate Reformist influence early on when consolidating power.
3. Do not pursue Reformist doctrines at a rapid pace early on as this placates Reformists. Early in 1978, you'll receive an event titled "Doesn't matter if a cat is Black or White" where Reformists who are disgruntled with the slow pace of reforms and, if they are influential enough, will demand a meeting and want you to start market reforms. In this event select the option to transfer power to the Reformist faction and their current leader, which if it's Xiaoping, will become the head of the country.

Thanks! I was able to do it! Maybe now I’ll get the “historical ending” achievement…