Which god of Japan is actually portrayed by Yato in Noragami? As far as I watched, Bishamon and Kofuku are portrayals of the Seven Lucky Gods (七福神 shichi fukujin). But it wasn't clear about Yato. Bishamon mentioned him as a God of Calamity, but there is no such god among the Seven Lucky Gods. So, which god might Yato be a portrayal of? Is he a new character altogether?
Kofuku is not the goddess of luck, and as explained in the series, she is the goddess of misfortune. She is not a part of Seven Gods of Fortune. The Seven gods are Hotei, Jurōjin, Fukurokuju, Bishamonten, Benzaiten (Benten-sama), Daikokuten (Daikoku) and Ebisu. Her surname, Ebisu is shared with the god of fortune as he will appear later in the series or you can just read the manga (which are raws) to find out more.

As for Yato, well, I think it's obvious that he is a fictional character based on the images of the gods of war in Japanese mythology.