Coincidentally, I just started a re-canvas on a 1920 18' GS IF yesterday and it too has steel gunwale screws - #10 1 3/4. First I've seen. Some of the heads are shot so these will need replacing. I'll have to dull down the shiny new ones so they won't stand out.
Also interesting is that the build record says Model IF but they stopped listing that in the catalogue in 1916. The pictures in the 1916 and 1920 catalogues are the same as are the dimensions but the 1920 is just a Guide Special. And the price in 1916 for the canoe with open gunwales was $34 but had gone to $74 by 1920. I think that that would have been quite a jump in those days - probably had to do with the war.
I agree with the carriage bolt idea. I'll grind slots in them for a screw driver. The original bolts are swaged extremely, just below the heads to stop them from turning. The #10 steel screws are in remarkable condition for ninety plus years. No rust at all . They look as if they are plated. Did they galvanize back then?