He'd probably be very intrigued. Percy has a penchant for adventure and new things, and this fanfiction would be something completely different from what he's used to.
I think young Sasuke would have a very intense reaction. He was so consumed by his hatred for Itachi at that young age. A Sasuke vs Itachi fanfiction would be like a trigger for him. He would be eager to see how the fictional fight is portrayed. If Sasuke in the fanfiction is shown as strong, it might fuel his own ego and make him strive even harder to become powerful. However, if the fanfiction shows Itachi as superior, it could enrage him. He might feel that it undermines his own goal of defeating Itachi and he might even want to find the writer to correct their 'wrong' view.
Boruto's reaction would be complex. He'd be heartbroken, for sure. He might start to question his own worth and purpose. But over time, he'd realize that he has to carry on Naruto's legacy. He'd become more serious and focused. He'd also become more empathetic towards others, understanding the weight of sacrifice. He might even start to study Naruto's techniques and leadership style more closely, trying to emulate the things that made Naruto such a great Hokage and hero.
As of now, Naruto and Sasuke haven't died in the Boruto manga. Their characters continue to play significant roles and contribute to the story's development.
It depends on the context. If it's a battle of pure physical strength and chakra, Naruto might have the edge as he has years of experience and has faced more powerful foes. But Boruto has some unique abilities like his Jougan. So if the fight involves using new - age ninja techniques, Boruto could surprise Naruto.
I like Naruto in these fanfics because he always stays true to his beliefs. He is constantly trying to bring Sasuke back to the village and is willing to fight for their friendship.