Well Frank the Kanji 忍者 also has kun reading as well as the Hanzi used in China . Interesting since there are Koryu schools that have Ninjutsu in their curriculm. Ok Koga is an area, though correctly said as Koka, Yamabushi is a religious sect who in history may have had a small influence on ninjutsu, However there is no support that there is any Yamabushi sects that were called Koga Yamabushi ninjutsu, did something refered to as Koga Yamabushi ninjutsu, further more into the 20th century, it is highly doubtful that anyone who is practicing Shugendo has much information or martial application pertaining to ninjutsu. In most situations when naming a ryuha it has a person's name rather than something as odd as Koga Yamabushi ninjutsu. All I could find on an Tanaka Emikio is she lives in Nagasaki but there are quite a lot of people with that name so who knows. If it is prominent as you say, then in Koryu circles it should be well known as that is what prominent means. Well in Koryu the forms may not be changing however, Koryu does Goshinjutsu techniques that can be applied in modern times.