Well if a song is to be totally original then surely it should have a unique tune too? Yet many fine songs have been written to old pre-existing tunes, I will just quote the one, Yellow on the Broom as a good example.
I exclude however parodies from the originals category as they rely almost wholly on an old set of lyrics and tune for their existence.
So a song 'after the style of' or 'with acknowledgement to' an earlier writer is perfectly valid in my book.
Giok