Goku's victory over Hit in the manga was the result of his perseverance and the ability to surprise Hit with unexpected moves. Also, his experience in battles played a significant role.
Friendship is also a theme. Despite being rivals, they could develop a deeper understanding and friendship in some fanfictions. It might show them training together and sharing their knowledge about combat and different universes.
One possible plot could be that Goku's power development takes a completely different path. Without the head injury that altered his original Saiyan nature in the official story, he might be more in tune with his Saiyan instincts from the start. This could lead to him being more aggressive in his battles and perhaps having a different moral code when it comes to fighting. He may not have the same innocent and somewhat naive view of the world that he had in the original story, which was partly shaped by his head injury.
The different interpretations of the characters are great. In some fanfictions, Goku Black might be portrayed as even more evil or cunning than in the original series, and Hit could be shown with new strategies. For instance, a fanfiction might have Hit teaming up with other characters from different universes to take on Goku Black, which adds a whole new dimension to the story.
Well, Goku's power level progression could be different in that he might be more in control of it. Since the head injury had some impact on his psyche, without it, he could have a more calculated approach to increasing his power. He might not rely as much on the Kaioken technique, which was developed in part due to his need to overcome limitations caused by his injury - affected development. Instead, he could focus more on pure Saiyan power - up methods like the Great Ape transformation and evolving it further.
There's also a fanfiction called 'Hit's Vengeance on Goku Black'. In this one, Hit is on a mission to avenge the destruction Goku Black has caused in the multiverse. It has some really intense fight scenes that are well - written. The author does a great job of portraying Hit's calculating nature and Goku Black's menacing presence.
Some Dragon Ball fan communities on Facebook or Discord may also be a good source. Members often share their favorite fanfictions, and you could specifically ask if anyone has any good Hit x Goku ones. Just be aware that not all communities may be as active or have a large collection of fanfictions.
If Goku never hit his head, he might not have become the kind-hearted hero we know. Maybe he would have remained a fierce warrior focused only on power and conquest.