While I wouldn’t want to discourage you from engaging in such an admirable task, I would caution that producing anything approaching what could properly be called a biography of Yang Chengfu might prove to be a very difficult endeavor requiring an enormous amount of work. Do you intend to use Chinese sources? I ask because, as I think you’re beginning to discover, the sources in English on Yang Chengfu’s life are extremely meager, and tend to be more anecdotal than historical. I suspect that this is the case in Chinese sources as well, but delving into Chinese sources would at least give you a better grasp of his working associations, his involvement in early martial arts and physical education study societies and the like. I would begin by carefully studying the many Chinese sources appearing in the bibliographies of Douglas Wile’s Lost T’ai-chi Classics from the Late Ch’ing Dynasty (SUNY, 1996), T’ai Chi’s Ancestors (Sweet Ch’i Press, 1999), and Barbara Davis’ The Taijiquan Classics (North Atlantic Books, 2004).
It occurs to me that, for a start, a more manageable project may place more emphasis on “his influence in the world of taijiquan” than on “his life.” There are a potentially a lot of ways of going at the topic of his influence, but unfortunately a detailed and reliable account of his life might be a frustrating and years-long undertaking.