@WBYeats,
Quote: I wanted to say the poetry of Old Japanese, not Japanese poetry that is old
Aha. I see.
But I don't think that makes a great deal of difference.
It's clear enough when you write "Old Japanese", and not "old Japanese", that you are referring to the older language.
Again, you could expand it by saying something like " the poetry written in Old Japanese"....."in the entirety of poetry in Old Japanese"...."in all poetry in Old Japanese".
There are a number of ways you could say it, and as you can see, the "the" is not necessary in some of the examples I have given.