A Clockwork Orange would be very difficult to adapt into a film. The novel is filled with so much detail, and trying to figure out the dialogue can sometimes be like trying to solve a puzzle. Filmmakers would definitely have a hard time writing the script, in a way that the dialogue stays true to the Cockney Russian dialect that young Alex speaks yet is able to be understood by an audience watching the film. Filmmakers would also struggle with the violence of the novel for a film. It would also be tough for actors to learn the accents and dialects. The first part of the novel is Alex's moloko plus (milk spiked with absinthe) induced adventures as he commits ultra-violence, and it would be very difficult to cast someone who would have to act out violent brawls and intense rape scenes, because very few actors would want such a role.

The first scene that is essential to a Clockwork Orange film would be ultra violence in part one. At the height of their rage, Alex and his droogs Pete, Dim, and Georgie break into a house in a village outside the city and rape a woman in front of his wife. As terrible as this is, this scene is very necessary because it is the most raw and coarse way to show Alex and his droogs' total disregard for people, and the recklessness of their youth. Stanley Kubrick includes this rape scene in his adaptation, and probably directed one of the most disturbing scenes in film history. This definitely gave people a visual of Alex's crave for orgiastic violence.
Another scene that should be in an adaptation is the brainwashing that Alex volunteers for in prison so that he may regain his freedom. During the brainwashing, doctors condition Alex's body to become sick and ill whenever he even has the thought of committing violence. They do this by giving him chemicals as he is forced to watch extremely violent acts while his eyes are held open by contraptions. This is very important, because it leads to Alex's inability to free will, which is very important to the title "Clockwork Orange". This is used very well in Kubrick's adaptation, and is quite disturbing, seeing the way that even a violent person like Alex is feeling tortured by being forced to watch pure violence.
The last scene that needs to be in a film is after Alex's conditioning has wared off, and he has regained the ability to commit violence with new droogs. Later Alex is thinking about starting a family, and he has been less prone to violent acts. I think this is necessary because people need to see that Alex has the potential to change without the conditioning, and that he is capable of sweetness and juice without being programmed and mechanised like a clock.

The scene that I would not include in the novel would be when Alex drugs and rapes a pair of girls that he says look no older than 10. Kubrick includes this scene in the film, but the girls are much older than 10, and consenting, so Alex doesn't rape them in his adaptation. I can't really think of a filmmaker who would include that part of the noel verbatim. I would probably change up the circumstances like Kubrick did.
I would omit most of the scenes that Alex speaks with his parents. I would include them a little bit, but not a ton. Without the audience seeing Alex's parents, I think it would help with the mysteriousness and the lack of morals that Alex has, because his parents are rarely seen. I omit the part where Alex returns to them after prison to suprise them, and finds out that someone else took his place on the family. I found that part in the novel sort of irrelivent and did little to the plot.
The last scene that I would omit would be Alex being cured and actually deciding to leave violence behind. Kubrick's adaptation does this as well, and it leaves the audience wondering what Alex is going to do with his life, and if he'll continue with violence or choose a more righteous path.

"It would sharpen you up and make you ready for a bit of the ol' ultra-violence."