You can often find such fanfiction on dedicated fanfiction websites. Some popular ones include Archive of Our Own. Just search for 'Young Xehanort and Eraqus' on the site, and you'll get a list of relevant stories.
I'm not sure if there is an exact 'naruto is xehanort fanfiction' out there. But if it existed, it might be a really strange and creative crossover. It could potentially involve Naruto somehow getting involved in Xehanort's schemes or world, maybe Naruto's powers being used in the context of Xehanort's story. It would be a very wild combination of two very different fictional universes.
Power is a common theme. Xehanort is often a power - hungry character, and in crossovers, his pursuit of power is pitted against the powers and rules of the new universe he enters. For example, if he crosses over into a universe with a strict moral code regarding magic use, his power - hungry ways will be challenged.
A fanfiction might focus on a journey they took together. They could be on a quest for a powerful artifact. Along the way, they face various challenges that test their friendship and their understanding of the Keyblade. For example, they might encounter a group of powerful Heartless and have to combine their skills to defeat them.
One great place to start looking for Master Xehanort and Eraqus fanfiction is on fanfiction.net. There are often some really creative takes on their relationship there. Some fanfics focus on their early days as students, exploring how their different personalities began to clash and also connect.
The clash of different power systems would make it interesting. Naruto has his chakra - based ninja techniques, like the Shadow Clone Jutsu and Rasengan. Xehanort has his Keyblade and his dark magic. Seeing how these powers interact, whether Naruto can use his speed and clones to outmaneuver Xehanort's attacks or if Xehanort's magic can nullify Naruto's jutsu, would be exciting.
One idea could be a crossover where the spiritual energy from Bleach somehow intersects with the magic in the worlds related to Yen Sid and Xehanort. Maybe a character from Bleach gets transported to their realm and has to navigate the complex power dynamics between Yen Sid's magic and Xehanort's schemes.
One key element is the power struggle. Xehanort is often portrayed as a powerful and somewhat menacing figure, and Ventus has his own strength and will. So, the fanfics usually show how they interact in terms of power, like Xehanort trying to manipulate Ventus's power for his own ends.
If you're part of any Kingdom Hearts fan communities on social media, like Facebook groups or Reddit subreddits, you can ask for recommendations. Fellow fans are often very helpful in pointing out hidden gems of fanfiction about Master Xehanort and Eraqus.