It's like no one here ever heard of the Karate Bullfighter Trilogy!

That was the first series of films about Mas Oyama starting in 1974. Those films are far more outrageous in contrivance than this one, but they starred Oyama's student Sonny Chiba and Oyama lived for another two decades following their release. Thus, I think one should consider that earlier film series as "approved" biographies -- preposterous though some of the events depicted were.