The reference is to A Clockwork Orange. Alex is the leader of a small gang, and he loves Beethoven’s Ninth Symphony. By bad luck, this is the music that is played in his aversion therapy, so he can’t bear to listen to it anymore.
As to the question, I don’t think Mozart would have worked for the story.
... or Bach!
Oh — I haven’t seen A Clockwork Orange since it first came out — that will tell you how old I am!
Mozart wouldn’t have worked in that film. But Mozart wrote a most magnificent requiem! And lots of other sublimely beautiful stuff.