Substitute Demerara Syrup for Demerara Rum?

I have Lemon Hart 151 on hand but, no way to get Demerara syrup or sugar within a reasonable time frame.

Would this substitution work? And if so, any proportion changes (i.e: recipe asks for 1 oz Demerara syrup, would I use 1 oz of Lemon Hart, or more, or less)?

Edit: For all the "that's like replacing X with Y", I'm just trying to understand this whole tiki cocktail thing. I literally know nothing about rum, liqueur, etc. Apologies I assumed that two things with the same name would have similar flavor profiles.

Much appreciation to those with suggestions, links, etc. Yall are awesome!

As for "experimenting," the 151 definitely is not a substitute, way too overwhelming. As pointed out, flavor notes are similar, but no sweetness and way too strong. Made a brown sugar 2:1 syrup (found online as a close substitute), bit too sweet for me. I'm going to try to find demerara sugar to make it properly.