Oh yeah, I remember that one!
I actually remember the plot twist being one of the weaker parts of the narrative. Going in blind, I was expecting it to show up somewhere at the end, but didn't find one... It definitely went over my head. To me it just seemed like a rather natural and typical conclusion for this kind of story. I don't think you have to change anything else except for the twist; And I'm interested in helping you out with it.
I've already read the novel, would you mind telling me what the twist was supposed to be, under a spoiler warning? I think I can work from there.