Based on my understanding (which isn’t much, please mention any additional things I missed) Marx believed that the “proletariat” (the workers) were being abused by the “bourgeois” (the owners) in the capitalist system, and that the proletariat should seize control of the state and the means of production (“dictatorship of the proletariat”), and that the end goal was a stateless, classless society where everyone was equal, and that the state would “wither away”.
As we all know, a perfect communist society was never achieved, and that the state never ended up withering away for any of them.
How would Marx react to the Soviet Union under Stalin and his purges, Khrushchev to his denouncing of Stalinism and brutal crushings of protests in the Warsaw Pact states, to Gorbachev and his “glasnost and perestroika” reforms?
How would Marx react to the communist states that took power in Latin America, Africa, and Asia? Would he be happy that a communist state was able to compete with the capitalist U.S. in terms of global dominance, twice (Soviet Union during the Cold War, PRC in the modern day)?
Note: I am neither procommunist or anticommunist. I think that some if Marx’s ideas were quite good (everyone should be equal, classless society, etc.) but others not so much (history tells us what happens when there is a “dictatorship of the proletariat”, the state never withers away like Marx imagines it would, as power corrupts all)
It’s important to understand that 20th century communist states weren’t just “communist” (there’s no such ideology as communism); they were Marxist-Leninist, which despite the name is a rebranding of Bolshevism by Stalin. “Socialist” and “communist” are incredibly broad terms, and the idea that communist = implementing Marx’s ideas is so reductive as to be just wrong. Now Marx’s opinion would likely vary depending on time and place, but at least he’d probably condemn Stalin’s USSR as an authoritarian hellhole. Beyond that I have no idea, but many Marxists who were contemporary to the things you describe condemned them and many others supported them, so we can’t make a realistic guess without projecting our own values on him. Basically what you’re asking is analogous to “what would Adam Smith think about the current state of the US;” it’s something we can speculate about but generally isn’t as salient a point as seem to you think it is.
PS: I suspect you don’t know much about Marx’s ideas, so you should start from there. First, the dictatorship of the proletariat isn’t necessarily an actual dictatorship (that’s not how the term is used by Marx).