Well, can you name some?
> intellectuals did with Marx himself) won't make it go away. There is
> already a vast body of scientific theory and scientifically based
> practice of economics, politics , history , law , even literature.
> Marxism has a full critique of positivism.
The sense in which Marxism is "scientific" is much more complicated than you are suggesting here, though. I don't see anything in Marx which looks like a theory in the natural sciences, that is, a precisely (probably formally and mathematically) specified set of relationships between observable phenomena. What I do see in Marx is a generally systematic attempt to give an account of the relationship between the appearances and essences of certain phenomena, which employs both empirical investigation and philosophical concept-creation. Which you can consider scientific in a general sense, but it's such a general sense I don't see why it wouldn't also include Foucault (in any event setting up an opposition between Marxism and Foucault seems untenable to me - Foucault was one of the later 20th century's greatest Marxists).