Hi Thomas
The main gendai JMAs (the modern ones ending in -do) do not have any equivalent concept of nei gong / internal power at all. The exception is aunkai, which is based around it.
However, if you look into the old style schools / koryu, there are hints to be found, but they tend not to talk about it too much in the open.
You can find it in:
Yagyu Shingan Ryu
Takemura-ha Shindo Yoshin Ryu (check out Toby Threadgill)
the schools of Kuroda Tetsuzan
Daito Ryu, especially the Sagawa (quite secretive) and Kodo lines (check out Roy Goldburg)
I was told about other koryu that have some internal stuff too, so the above is not exhaustive.
Internal power in the Japanese arts came from China hundreds of years ago, so there isn’t a huge variation in the tools used to train it. One key difference is that for some reason the Japanese arts, in my experience, have more limited vocabulary: you can’t easy discern between the hips and the waist, which is key to separate for IP, and alo the Chinese concept of kua, again it’s taught as movement, but I don’t know of any Japanese word for it.
Hope that helps
Dan