I know Hegel greatly influenced Marx, but have any others been important in the philosophy of Marxism?
Bakunin. You enemies have to influence you somehow, right?
https://m.youtube.com/watch?v=0AwdihHME1M This podcast episode is good. You can probably skip the first 5 min. Episode 80 & 82 are good too.
Aristotle, his ideas of republican catagories and science are all over Marx, as well as stuff like species being.
German idealism like hegel of course, though marx studied under one of hegels students.
Rousseau somewhat also. The utopian Socialists as well.
The various different Utopian socialists, and the early anarchists come up a lot. Of course if you are looking for straight philosophy Aristotle is one, and it can’t hurt to be familiar with the general milieu of German Idealism.