aimixin talks about morality and Marxism:

Saying Marxism isn't about morality or excludes morality isn't meant to say Marxists are immoral or amoral. It's sort like, computer science doesn't talk about morality, but that doesn't make computer science immoral, or software developers amoral. They're just separate topics.

Marxism is meant to treat socioeconomic development as a material science. Biology and chemistry can inform doctors on how to make medicine and what medicine to prescribe people. But biology and chemistry themselves do not prescribe anything. Prescriptions require some sort of stated end goal, which is subjective.

Stalin says something similar in Economic Problems of Socialism in the USSR, where he points out that political economy is the study of objective laws of social development which are outside of the control of the government, that the government's policies are not equivalent to political economy as a science but are prescriptions informed by the science.

This is what Marx had to say on the subject.

Communists do not oppose egoism to selflessness or selflessness to egoism, nor do they express this contradiction theoretically either in its sentimental or in its highflown ideological form; they rather demonstrate its material source, with which it disappears of itself. The Communists do not preach morality at all.

They do not put to people the moral demand: love one another, do not be egoists, etc.; on the contrary, they are very well aware that egoism, just as much selflessness, is in definite circumstances a necessary form of the self-assertion of individuals. Hence, the Communists by no means want to do away with the "private individual" for the sake of the "general", selfless man. That is a statement of the imagination.

—Marx, The German Ideology

In some sense, you can argue there is a Marxist morality, but not from the perspective of subjective prescriptions, but merely an attempt to explain an objective origin to already existing morality. Such as, the origin of liberal viewpoints, which are heavily steeped in morality, clearly emanate from the capitalist mode of production. One could also argue a socialist society would produce a different kind of morality, but this would not be a prescription but would have to be demonstrated with evidence.

I don't think there is any reason to try and force morality or ethics into Marxism. Marxism does not need to be some all-encompassing worldview. It's fine to get your beliefs and views from other sources. I am influenced by many writers, many of whom are not Marxist. I don't get all my ideas from one source, I don't feel a need to somehow make Marxism all-encompassing.