If it were in my possession, I would scan it in sections and "knit" it back together. Scan at no less than 300 dpi. The dpi can always be reduced, but not enlarged with good results. I have worked with many oversized pieces with great results. This site: http://www.ourroots.ca
has a digital copy of Butternuts and Maple Sugar : some history of the old 14th of Lancaster Township, Glengarry County, Ontario (5th concession Lochiel) they have made available with free registration. They are trying to put Canada history on-line so it is available free to people. If you can get a good digital copy made they might be interested in helping you. Agree, never ever let the original out of your sight!