I wasn't sure about the taste difference. I assumed the Usukuchi was "lite" in the soy taste department and not necessarily the sodium.
Wiki says: It is both saltier and lighter in color than koikuchi. The lighter color arises from the use of amazake, a sweet liquid made from fermented rice, that is used in its production.
koikuchi is regular soy sauce also known as light soy, as opposed to dark soy sauce which is thicker and sweeter. 'ite' is reduced salt. I would just substitute regular soy sauce and taste for salt. Add a pinch at a time if it seems needed.