The original score of W. M. is shown on the ANU website. Maybe MacPherson's tune is different enough to be considered "original." The score includes the chorus: Original score

Neither website I have linked to mention Mozart in connection with the tune. The one linked here says it is not easy to pick out the relationship to "Cragielea."

Notice the comment about "copyright." The Cowan version is still © in the United States. There is discussion about that in the website (also discussed in one of the previous threads).