Wasn't Ronald Regan friends with Gorbachov? And he visited President Li Xiannian of China in a big historic event. I don't really think he was as anti-communism as most people think. He was pro-capitalism, but he seemed fine coexisting with USSR and CCCP. So long as they didn't have the advantage on the world stage over him.
So I doubt he would have engaged in operation unthinkable.
That being said, I'm a strange person, so the first answer to the question that popped into my head was "FDR/Regan immediately has some sort of anyrusim/stroke because his brain isn't formatted correctly, dies, and we get president Wallace, which kinda sucks balls because Wallace was a bit of a limp noodle from what I remember of history"
Busch was happy with soviets,and even try to save them/Kiev chicken speech/
But Reagan saw soviets for what they were - empire of evil.And would destroy them,with Manhattan program make earlier and without supporting soviets,they would fall quickly after Hitler.
In this timeline - about 1943-44,i think.
China - there he could let Mao take part of it,Czang another - and fight each other.Thanks to that,united China would be no problem for USA.