What would happen if Greece defaulted on western debt and aligned with Russia and China is certainly not a repeat of the fascist military dictatorship they had back then.
They most likely would have social-democratic governments in the next decades, exorbitant financial investment from both Russia and China and lots of Russian military bases on their territory.
This would most likely be the end of the EU (and NATO) as we know it, because from there Russia and Turkey would work on pushing the EU out of the Balkans, which would most likely work considering that all of the populations there have a strong preference and historic ties to either Russia or Turkey, depending on what country you look at.
Even in Bulgaria, which is currently in the EU and has a pro-EU government, about 80% of the population have pro-Russian sentiments regarding the conflict in the Ukraine.
This is why the EU and US will do whatever it takes to keep Greece in the EU, even if it means that they have to orchestrate a military coup in Greece or that they will have to print trillions to cover for the debt.