The info mentioned is located on the lower half of p127, the last entry in the book, and it doesn't mention any fire repairs. It is a photo of the North American, and the text is essentially what was reported above: "Although not built in Manitowac, the ships were frequent visitors to Manitowac for dry-docking and other repairs..."
South American is listed in the MHSD book "Great Lakes Ships We Remember II" on pp322-325. Here it says the rebuild after the 1924 fire was done in Ecorse at Great Lakes Engineering.
MHSD Journal V18-3-2 reports that in 1964 the South American underwent considerable refit at Manitowac including replacement of lower hull plates, new propeller shaft and wheel repairs. This might be where the plaque came from.
Erin, I live in St Ignace and I have these books if you need to reference them. I also have several DVD's on the Georgian Bay Line, plus one by MSRA "The SS South American" I haven't watched them recently so I don't recall what information they may be helpful. There may be further information contained in Inland Seas, more MHSD Journal archives, Telescope, or other sources in my collection but it will take some time to review all these materials. I'll stop in soon to say hello