Well, it depends on what you mean by 'good'. There could be some out there, but it might be a bit hard to find. Some fanfic writers might focus on Brian Toreto's childhood experiences or have a child character interact with him in an interesting story. However, not all fanfic is well - written, so you might have to dig through a few to find one that suits your taste.
One common theme could be family. Since Brian Toreto is part of the Fast and Furious family, a fanfic about his childhood might explore his family relationships. Another could be friendship, showing how he made friends early on.
Well, one major difference could be the way the characters are drawn. In the manga, there might be more detail and specific styles that don't always translate exactly in the animation.