In 1883, Karl Marx wrote a letter to Paul Lafargue and Jules Guesde, the French Marxists, and accused them of denying the importance of reformist struggles. Marx had become more sympathetic to parliamentary politics as a tool for bettering the condition of workers and began to realize the importance of trade unionism for ameliorating the woes of the proletariat; and had begun the revision of Marxism that would ultimately be completed by Eduard Bernstein. He was headed in the direction of social democracy, though Marx would have never settled for a mixed-economy solution to the problem of capitalism. In that letter, he asserted that insofar as the French socialists actually represent the ideas espoused in "The Communist Manifesto," he could not identify as a Marxist himself. This is the origin of his famous line, "I am not a Marxist." What he meant is that he no longer thought that the revolution necessarily had to be violent. He believed that electoral politics and reform could achieve socialism in places like America and Britain.
I don't think Marx would have settled for the "mixed economy" that followed from Fabian Socialism and Bernstein's evolutionary socialism, nor would he have liked the fully-socialist Lange-Lerner model of market-socialism. He probably would have still pushed for the abolition of money and markets—he would likely have continued to be a communist in addition to being a socialist, but he was becoming more open to the idea that the revolution could be peaceful. Unlike Bernstein, Marx would also have continued to demand public-ownership of the means of production too, rather than settling for a watered down mixed economy.