I'm not sure specifically which "motorcycle headbutt" story you are referring to. There could be various incidents involving motorcycles and headbutts in different contexts, like in a motorcycle race where a rider might accidentally collide and there could be a headbutt - like impact. Without more details, it's hard to tell the full story.
Yes, in some fanfictions, Tanjiro is depicted as the reincarnation of Yoriichi. This concept adds an interesting layer to the story as Yoriichi was an extremely powerful and legendary figure in the original lore. The idea allows for new storylines where Tanjiro might inherit some of Yoriichi's skills or have a deeper connection to the past events related to Yoriichi.
Tanjiro's fate in the manga is that he ultimately passed away. This event was a significant part of the story's development and had a profound impact on the other characters and the overall plot.
Tanjiro's strength in the manga is significant. He acquires new powers and techniques, and overcomes numerous powerful foes with determination and strategy.