IIRC, the Koto voice is an old, old one. Not in the same league as a S.Art or Cool! voice.
One of the exam pieces used to call for oriental sounds, including Koto. It's a been a year or two since I last taught it to anyone and I think that, even on the E series keyboards, you could layer the Koto with one of the other oriental stringed instruments. Shamisen was one of them.
You could also experiment with adding a touch of chorus to the voice or voices used. Not much, just enough to give the impression of an instrument that's not perfectly in tune.