eh, I suppose i could and should have been clearer, I was being lazy, sorry.
We oppose those forms of morality that are founded on exploitation and capitalist relations. Generally speaking, I would say that referring to moral or ethical arguments isn't particularly useful when advocating the overthrow of capitalism - sticking to material explainations is usually the way to go. The reason for this is that moral arguments are usually subjective to some degree, and it is as easy to argue that it is morally right to struggle for communism from one perspective as it is to argue that we should reinstate kings and queens from another point of view*. I suppose that as far as current systems of morality go utilitarianism and kantian ethics have a lot in common with many communist principles, so if for whatever reason I did want to go for a "moral basis of marxism" those would be sensible things to resort to.
However, like a lot of people in this thread have said, a proletarian, communist revolution is not rooted in the same kind of principles as capitalist society. It is a far-reaching revolutionary movement which will involve a lot of fundamental changes to the functions of society. As such, a moral code (or perhaps moral codes, I imagine people won't stop debating what is right and wrong regardless of their conditions

) that will deal with the conditions of a revolutionary and post-revolutionary world will have to evolve in those kinds of situations so as to meet the needs of those conditions. To try and think of a concrete one now would be akin to someone a few hundred years ago thinking of a comprehensive moral code for the digital age - at best, literature will be produced :P
I suppose what i'm trying to say is that a moral code for a revolutionary, communist society will necessarily have to be born and grow up with that very same society.
*interestingly you actually deal with this issue in your opening post "Kantianism is often said to be the political philosophy on which Marxism is built (with a few tweaks here and there), however many on the right have also invoked Kantianism to support their posistions." Two fundamentally opposing groups can use the same (or at least very similar) moral arguments and formulas to back up their conflicting points of view.