Marx's achievement was in constructing a new way of understanding and interpreting history, not in the discovering a new ethics.
I would argue that there is no such thing – any more than there is a Newtonian ethics or Freudian ethics.
Marx’s achievement was the discovery of historical materialism: i.e. the development of conceptual tools which help us comprehend human history. And Marx himself was of course the first to undertake such studies within historical materialism. The construction of statements of fact which result from these studies cannot of themselves create ‘ought’ value statements.
Now of course Marx himself was passionately associated with a particular politics: namely that of moving towards a communist society, an ideal which was already formulated in Enlightenment thinking. Yet there is no theoretical originality in this: all Marx did was to take the results of his sociological analysis and argue: if communism is to be achieved then X, Y and Z ought to be done and happily for him what ought to be done was what he thought was actually happening in nineteenth century society. Hence Marxist politics was born, which involved an interesting interrelation between what is and what ought to be done.
Marxist politics has however re-entered historical materialism in one sense: the choice of value-laden terminology to describe factual states of affairs. For instance, the more value neutral concept extraction of surplus value is referred to by the value-laden term exploitation, implying that the extraction of surplus value, a fact of capitalism, is morally wrong.