The Garou OPM manga is the original source, and the anime adaptation does a decent job of bringing it to life. It might compress or expand certain scenes, but the core elements remain intact. Fans of the manga usually appreciate the anime's efforts to recreate the world and characters.
For a good character interaction between Garou and Saitama, start with their fundamental differences. Saitama is a hero who is bored with the lack of challenge, while Garou hates heroes. Have them start with a fight, but make it a comical one where Saitama's nonchalance annoys Garou to no end. This can lead to some great dialogue exchanges where Garou tries to taunt Saitama into taking him seriously, but Saitama just doesn't care.
Yes, there might be. The Naruto and One - Punch Man fandoms are both large, and it's not uncommon for fans to create crossover fanfics. In such a fanfic, Naruto could be depicted as a fan of Garou for various reasons, like being impressed by his fighting style.
One idea could be a story where Garou and Metal Bat are forced to team up against a common enemy stronger than they've ever faced. They start off hating each other but gradually learn to respect each other's fighting styles.
The plot might involve Naruto reading about Garou in some kind of multiverse scroll. Intrigued, he uses his shadow clone jutsu to search for Garou. When they meet, Naruto is all excited and asks Garou a million questions about his fighting techniques. Garou is initially annoyed but then realizes Naruto has a pure heart and they end up having adventures together.