If they leave the Euro Greece "can forget about running a capitalist economy" ?
They would certainly have to make an accommodation with China and Russia as well as navigate past the danger of a military coup, but I see no reason they could not have a considerable private sector. I think that both Russia and China are far beyond caring about any sort of pure communist government. The challenge would be having a government that was relatively free of corruption. That has been the failure mode for the last half century or more.
Provide the Generals with a screening of Peter Seller's The Mouse That Roared either with Greek sub-titles or dubbing if not all of them could follow the English script or it has not already been dubbed.
I think it is less a question of capitalist economy as in private ownership of the means of production and more a matter of being a part of the international sphere of capitalist economies, ie the west.
And it is a good question. We know that non-compliance is punished with financial blockade. If a country like Greece leaves the euro, we don't know what actions will be taken. Blockade is illegal according to at least EU and WTO rules, but just as with the financial blockade the purpose is to topple the government or change its course so fast that there will be no time to sue.
So while Greece does not know if there will a blockade, color revolution etc, it does best in preparing for such options. Which boils down to going Cuba in self-reliance or making sure to have a lifeline in Russian support.
And convincing their generals they will be exchanging their western weapons for new Russian weapons that are as good or better and also cheaper - true or not - and that the alternative is to be bled dry by a pack of cowards who are supposed to be your allies acting in the interest of Germany and other EMU surplus countries and being unable to defend yourself no matter how many shiny western weapons you have.