Wow, there's actually Shura no Mon fans here. I remember reading that series a long time ago, and looking back, I'm really impressed to how Kawahara kept up with each era of fighting sports and martial art styles and its impact on mainstream culture.
The Shinbukan arc definitely deals with Kyokushin karate as its homage. The God of Martial Arts, Ryuuzoji Tesshin is Sosai without a doubt.
Takemi Naoto probably relates to Toshio Fujiwara; can't really think of any other famous Japanese Nak Muay it can relate to. Hayama Satoru is probably an amalgamation of Satoru Sayama (Tiger Mask) and Cesar Takeshi of Shootboxing.
Hida Takaaki is obviously modeled after pro-wrestler Antonio Inoki. His trainer, Frank is a nod to the God of Wrestling, Karl Gotch, who brought pro-wrestling to Japan in the 1950's.
Arios Kilrein is Mike Tyson...lol, there's glaring similarities like both losing their trainers and going down the wrong path. Arios even uses Tyson's peek-a-boo style that gave everyone (especially Tsukumo) a hard time.
And you can't forget the Bob King-Don King parallel...and the objection to the referee for counting to fast. Buster Douglas-Tyson at Tokyo anyone?
And then the Vale Tudo arc...this was written in 1994, right around the time that the UFC and Brazilian jujutsu was making a real impact in the United States.
Quite a bit of the participants of Vale Tudo were ficitional representations of their real-life counterparts in the UFC and K-1. As kakatootoshi mentioned earlier, Ignacio da Silva is most likely inspired by Franciso Filho. William Smith's character clearly resembles the look and attitude of Pat Smith. Ramon and Leon Gracielo (obviously Royce and his brother Rickson).