Hi, Not researching this family - but a thought.
You know that John and Catherine McGREGOR - came to Canada 1831 Horton Township 1831 Do you know if Children were born in Scotland or Canada -
When was the first child born - that could give you a basic idea when the marriage took place
Depending on When John and Catherine McGREGOR passed away - perhaps obituaries had been published- perhaps family papers - photographs can help identify the parents
Did other family members also come to Canada.
Have you posted to Message Board - in Scotland -
the following might be helpful
http://www.genuki.org.uk/big/sct/http://www.genuki.org.uk/big/sct/PER/Scotland Church Records
http://www.genuki.org.uk/big/sct/ChurchRecords.html