I recommend you have a close look at the keyboard layout to make sure you comfortable with things like the location of @, the missing `/~ key at the upper left, location of '/", /?, etc.
I don't think the Kana keys have any function unless you use using the Japanese IM, which you would not be: You should set your input source to US.
I have not heard of any way to reassign them, but it might be possible using the karabiner app.