There were two Peter MacKay's I know of that were associated with this family, but I don't think either of them went to Newfoundland. John, father of Peter at #1, below, was born about 1790 in Rum, and appears to have been the son of another Peter, b abt 1757, also in Rum, son of Donald MacKay and Janet/Gunnet MacKinnon.
1. Peter, son of John & Flora/Ann MacKay b 1820 in Rum, Scotland d 1890 Chimney Corner, Cape Breton had a wife called Margaret and sons called Edward, John (b 1848 Margaree Harbour), and Neil (b 1853 in Cape Mabou).
2. Peter b 1885, son of John b 1848, above, and Mary MacKay, daughter of Donald & Isabella MacKay.
The other John, son of Hector, who was in the 1818 census, doesn't appear to have had any sons or grandsons or great-grandsons called Peter.
Sorry I can't help more.