When writing a Star Wars Godzilla fanfic, the characters' interactions are key. If you have Luke Skywalker encounter Godzilla, Luke would likely be both awed and concerned. He might try to reach out with the Force, but Godzilla's mind could be too alien. And for Han Solo, he'd probably be more interested in finding a way to make a profit off this strange new creature, or at least get out of its way.
For Godzilla in the Star Wars universe, you need to keep his essence. He's a powerful, almost indestructible force. So in the fanfic, he should act like he does in his own movies - stomping around, using his atomic breath. As for Star Wars characters, they should react realistically. The Jedi might be cautious, sensing his power. The Sith could see him as a tool or a threat.