You must be more Japanese than me! Let see ..., as far as I looked up the dictionary. It says; WAGAKU and KOKUGAKU are taken the same. It's said that KOKUGAKU was established in the Edo period. Norinaga Motoori and Mabuchi Kamono are well known scholars who studied and developed widely in the era. But there is a funny story that WAGAKU or KOGAKU (the old study) were popular words to use in those days. And It's said that the word KOKUGAKU has been popular to use since the modern period. It's translated "the classical Japanese literature" as you mentioned. KOKUGAKU is the subject to study Japanese's mind and sprit from some classical literatures such as "Kojiki", "Manyoushu" and so on. Old scholars tried to find what really the Japanese spirit was. That's why they sometimes criticized some religions brought from China. It's said the word WAGAKU was used to separate from KANGAKU(the study of China) in those days. Today's Japanese people understand KOKUGAKU was studied in the Edo. And WAGAKU is the study with many Chinese ideas and it was studied before the Edo. And Norinaga Motoori and Mabuchi Kamono were more a poet than a scholars. They are really WA people. Sorry, my help is limited. I hope you find someone a historian.