The villains in One Punch Man might also have interesting reactions. Boros, if he could watch a fanfic, might be confused at first. He's used to his own view of the world and his place in it. But then he might see the fanfic as a chance to understand his enemies better. He could also be a bit angry if the fanfic makes him look weak or stupid. Overall, it really depends on the character's personality and their relationship with the original One Punch Man story.
Yes, there likely is. The Naruto and One Punch Man fandoms are both quite large, and it's common for fans to create crossover fanfictions. These crossovers can be really interesting as they combine the unique elements of both universes, like Naruto's ninja skills with Saitama's one - punch power.
I'm quite sure there exists a 'Naruto is One Punch Man' fanfic. The world of fanfiction is vast and full of all kinds of creative crossovers. Writers might be intrigued by the idea of bringing Naruto's ninja skills and the unique world of One Punch Man together. For example, how would Naruto react to the overpowered villains in One Punch Man's world? And how would Saitama view Naruto's determination to protect his village? These are the kind of things that could be explored in such a fanfic.