Honey is your real binding agent here, not maple syrup. You can substitute the sugar-free syrup but be aware of what's in it - according to the ingredients, it's mostly Sorbitol, which is both a humectant and a laxative.
Now, ordinarily, I wouldn't tell people to worry about the digestive effects of sorbitol, but that's a seriously large quantity you'd be adding, so if any of you have sensitive stomachs, beware of eating too much of that trail mix at once. The other property - humectancy - means that what you end up with is going to be moister than what you'd get with pure maple syrup. That may or may not be a good thing depending on your tastes.
Since your issue seems to be a lack of availability rather than a problem with sugar itself, I'd personally substitute generic pancake syrup, sugar syrup or even corn syrup instead. Or, if you're really keen on the sugar-free stuff, you can use less of it and add more honey because the sorbitol will help keep the whole product moist.
If you really want to cut down on the sugar, you can also use agave nectar, which is a fairly close substitute for honey (a bit thinner), and could also stand in for maple syrup if used in combination with another syrup and perhaps some maple extract or maple flavouring. It won't be the same flavour, but, texture-wise, it should be a pretty close match.
Best Answer
If maple is intended to be a significant flavor in the cake (i.e. you're making a maple cake, or a maple rum cake) it's not going to be the same without it. The best substitutions would be maple syrup, or white sugar plus maple extract, so that you still get that wonderful flavor.
As for quantities, looking around online, the common recommendations for replacing 1/2 cup of maple sugar seem to be:
If you can do without the maple flavor, then you could just use the 3/4 cup white sugar, but I suspect it wasn't a superfluous ingredient!
In the case of this recipe, it looks like the maple is important, but there aren't liquids to reduce. That probably means that the easiest option is the maple extract, but you could also take the recommended amount of maple syrup, and boil it down until you've lost the right amount of water. (For example, for 1 cup of maple sugar, take 2 cups maple syrup and reduce it to 1.5 cups.)