I'm not entirely sure specifically what 'chiho visual novel' is about as it could be a rather unique or less - known one. It might involve a story set in a fictional world created by the developers, perhaps with characters named Chiho and various plotlines like adventure, friendship or mystery.
Yes, there are likely fanfictions out there with such a concept. The Naruto fandom is vast and creative, and crossovers with historical or other fictional characters like Sasaki Kojiro are not unheard of. Some fans might be intrigued by the idea of Naruto having some sort of connection or admiration for Sasaki Kojiro and create stories based on it.
First, check the official websites of visual novel developers or publishers. They might have information about where to obtain 'chiho visual novel'. Additionally, some specialized gaming forums or communities might also provide leads on where to find it. Maybe someone in those communities has already played it and can give you the exact source.
One main theme is the clash of different worlds. Maou is from a different realm and now has to exist in the modern world with Chiho. Another theme is friendship or relationship building. Chiho and Maou develop a special bond throughout the story.
Hey, let me recommend a Doujinshi novel by Go Soul to you. It's called Go Soul: Onmyoji System." The novel was mainly about a person named Light who transmigrated to the world of Go Soul and met his childhood sweetheart, Fujiwara Sasaki. The two of them went through many hardships together and finally got together. The author of this book was Jun Wushang, and the quality was quite good! I hope you like my recommendation. Muah ~
Since I haven't actually read a specific 'maou and chiho lemon fanfic', it's hard to say exactly. But generally, it might involve some romantic or tense interactions between Maou and Chiho with some added elements of drama or adventure.