taking this date from their last born child Catherine born 1840 and died at sea
At ScotlandsPeople there is a birth/baptism for Catherine McKenzie with parents James McKenzie and Margaret McPherson in Loth dated 2 Nov 1835.
Where did you get the information about Catherine's birth and death?
I am entering this on Rootschat in the hope that someone might come back to me from Canada and be a relative ..I know they lived in Gore of Lockaber and Thurso Ottawa and that
Margaret died and James remarried and went on to have another 4 children Barbara ,John Donald and Margery
There is an 1861 census entry in Lochaber for George McKenzie with a wife named Anna and son Wallis. On the previous page and the top of the same page there is a James McKenzie with wife Margaret and children William, Barbara, Donald, James, Margaret, Janet, Robert and Alexander.
On the 1871 census for Lochaber, Margaret (listed as a widow) is there with children James, Margaret, Janet, Robert, Alexander and Charles.
I see James McKenzie and Margaret Campbell were married in the St-Philippe-d'Argenteuil parish on 22 Mar 1852. The marriage took place in Chatham, Ottawa River, Canada East according to the entry.
Were both of James' wives named Margaret?
Looking at the find a grave entry I quoted above it says Margaret was Margaret Campbell MCkenzie born 1822 but you say your Margaret was Margaret McPherson 
So although George seems to fit, it the wrong birth mother.
Are you sure that James married Anna Thomson The son George McKenzie was married to Anna Thomson.
Margaret Campbell was James' second wife so I think that must have been a typo saying it was Anna.
Jacquie
