I hate to be a killjoy, but stopping Marx from publishing his Manifesto won’t really change much.
Marx had many peers, among whom Engels, who could continue his work, and the less-famous, but very active at the time revolutionary Louis Blanqui, who had his own school of thought, which could still be called socialist.
And even before Marx, during and after the French Revolution and the Enlightenmen, there were authors, such as Charles Fourier, who themselves had a very detailed vision of society and how it should be.
Fourier believed that the most fundamental right is the right to work. Men should be able to work, not because of a primal desire guiding them, but because of their own will, free of opression of starvation and suffering.
He was also a social philosopher, believing in the liberation of passion in man. This is the basis of his work, really. He fancied himself a Messiah, who figured out the fabric of human mind and how to set us free from the divine punishment imposed on us after we tasted the Fruit of Knowledge.
You can read more on him, if you are curious, on here.
Amongst Fourier’s more famous peers was Saint-Simon, who also was a revisionist when it came to society. He called for a world, based on science, where an industrial/working class would lead mankind to prosperity.
There are also the old-school thinkers, such as Rousseau, who rejected the ideas of liberalism and believed that the root of evil can be traced to private property and unequal distribution of wealth. This line of thinking can even be seen in the Jacobins, albeit to a much lesser extend. You can read more on that here.
You may also notice that a lot of those philosophers base their beliefs on religion, especially the Bible. So you can also track the roots of socialism, which is empoworing the powerless and less fortunate, to the most influential religious text ever.
You can find ideas of collectivisation in Ancient Rome as well, where there were politicians favouring the Plebeians and distributing wealth from the rich to the downtrodden.
While Marx can be called the most influential socialist, it is without a doubt that his work is merely a continuation of the work of those, before him. This is not to say that he doesn’t deserve credit for what he wrote, but his thoughts are not unique and irreplacable. Socialism would have taken roots in the old feudal society of Europe one way or another. The roots of collectivism were always there. Someone just needed to pick them up.
Of course this isn’t to say that you cannot do this. In fact, I am looking forward to how you will change history. I just thought that this is interesting and worth writing down.