Banana Fish is not based on real events. It's a fictional narrative that takes the reader/viewer into a made-up world with its own unique plot and characters.
Banana Fish is purely fictional. The plot, characters, and events were all imagined by the writer to create an engaging and unique narrative. There's no real-life inspiration behind it.
In 'banana fish short story', it focuses on Ash Lynx's journey. He is a character in a rather gritty world. The story may include his relationships with other characters, like Eiji. The plot is centered around the mystery of banana fish, which might be some sort of drug or something more sinister. Ash is constantly in danger as he tries to uncover what banana fish really is and how it affects people.
I'm not sure there are many specific fanfics just on this one aspect. However, you could start by looking on fanfic platforms like Archive of Our Own. Search for 'Banana Fish Eiji kidnapped' and you might find some relevant works. Some fanfics may explore different motives for the kidnapping, such as using Eiji to get to Ash or for some political gain within the complex world of Banana Fish.