I find this question interesting, mainly because most see it as impossible.
I for one, disagree with this and I truly believe with a POD beginning with the creation of the Soviet Union that they could win. I also, don't believe that they would need to adopt capitalist reforms in order to survive.
I also think that, if any country could become and remain truly socialist it would be the USSR. They have plentiful resources, after all.
So with a POD in 1921, and without them adopting capitalist reforms, make the USSR win (or at least still be around today.)