Cait McPhee has identified the illegitimate daughters, Jean and Margaret Mutch, fathered by John with Jean/Jane Leggat. Cait is descended from Margaret and has confirmed the link to John through DNA. She wrote:
"J71 John marries Isobel Mackie and I think fathers Jean Leggat's children (from whom I am descended) as well, before dying in 1847 in Blackhills, Peterhead. I have the parish record of his death including the location.
In 1851 Isobel is in the census as "Widow Mutch", still in Blackhills, Peterhead, with children including John Murray Mutch, from whom I can trace DNA cousins, among others from this lineage.
Margaret Mutch was born in ~1836 in Cruden and in the 1841 census is living with her sister Jean, her mother Jean/Jane Leggat and her grandmother Jane Rennie. I can't find baptisms for either girl, but I did find mention of both of them in the kirk sessions: their father was a John Mutch, married man. John and Jean Leggat both got thoroughly told off - twice. Margaret's death certificate also gives John Mutch as her father.
My DNA results suggest that this John Mutch is the one that married Isobel Mackie, i.e. your J71, as I can trace cousins through both Margaret Mutch, my direct ancestor (who married a John Kemp), but also John Murray Mutch through the legitimate side of J71's family."