I love this movie a lot despite it's obvious low budget (The cross fades) So i've seen probably more times than anyone should have, and I started noticing what I consider to be allusions to the book that I suppose they couldn't include in the film for whatever reason.

The Homeless Man: It's not really supposed to be a scary scene, just add a bit of suspense but I guess it was probably a refrence to Eddies encounter.

The Opening Scene: One of the most famous scenes in the movie, at the end of it, we see the bike turned over with the wheel still turning in the air. This is the same description of 29 Neibolt Street.

The Shower Encounter: Eddies is in the school shower when he sees Pennywise, this is considered to be one of the scariest scenes in the movie. In the book during the dinner at the Jade of Orient, Mike discusses some of murders. Then he mentiones one where a child was in the bathroom of his own home and when the mom went in there, the shower door was smashed. Even though this isn't direct I feel like it could be a subtle refrence.

The Skeleton: Ben sees him in the Kenduskeag twice. I think this probably bares some resemblence to the Swamp Monster Eddie Corocan saw in the park.

I'm fairly certain there some more, tell me if you can think of any now that you understand my train of thought.