Hello Rikudou Sennin,
Here is my tiny understanding of the subject...
Yamabushi is essentially a term for mountain ascetic practitioners. It may have been something else in the past. Shugendo then is based in Tendai, Shingon, or Shinto practices. I don't know if there is a 'pure Shugendo' or a fixed tradition of mountain sages...since it was all developed out of so many different religions and shamanic-like practices. Other sects may also have their own variants of mountain asceticism. Each Shugendo tradition then has it's own variations, subsects, groups, temples it is associated with etc. Some larger, some smaller.
Are there siddhas? Yes, I am sure there people who have developed spiritual power. There are people who have mastered various aspects of Tendai practices. Tendai is very eclectic so it depends what practices you are looking for specifically. You can find people who are experts in all sorts of aspects of dharma practice.
See Marathon Monks of Mt.Hiei or the various documentaries of similar names online. They kaihogyo monks live closest to lifelong mountain asceticism. I am gonna guess there have been mountain sages since mountains were around. I also understand things like the yamabushi, kaihogyo, and shugendo developed out of people living ascetic religious lives in the mountains and practicing. Then it gradually became more organized.
If Tendai is your interest there are options in America you could contact. They could possibly help you to understand yamabushi within Tendai better?California Tendai Monastery
, A Tendai-Shugendo mountain monastery. - www.caltendai.org
Here you would be a renunciate monk engaged in ascetic mountain practices of both Tendai and Shugendo. You would practice dharma and life a live of service to others. It is also possible to stay and practice for intense shorter periods without becoming a monk. Tendai Buddhist Institute
in NY. www.tendai.org
They have a developed education program, spread over a period of years, that would prepare you for the various forms of service and community building expected from a Tendai priest. I don't know about residential/long term possibilities there but you could inquire or attend one of their short retreats.
You could also contact Rev.Ara in Hawaii
as he has trained individual priests. I don't know about his relationship or practice to mountain based asceticism.
If you are able to communicate in Japanese or already have a connection with someone in Japan you could go there and see with who and where you connect. Visit mountains and temples like Koyasan, Kinpusen-ji, Hieizan, Dewa Sanzen etc.
If you want to practice the dharma to serve sentient beings- there are definitely many places to learn it and do it. Many types of people who are masters of various aspects. If mountain asceticism in Tendai is your dharma door...then the internet is possibly the least effective medium for it. Best best is visit places!