Generally speaking, if you need to access a list by value, dictionaries are a good candidate for consideration. In this case, since you have an arbitrary list of objects associated with a quantity, the first idea that comes to mind would be a dictionary where the keys are item IDs (perhaps constants the user never sees) and the values are quantities. I don't know if there's an idiomatic way to do this in Unity and/or C#, but as far as data design, it's the first thing that occurs to me.
It's worth the brief pain of updating the visualization scripts because it's far less likely you'll need to do that again, at least in terms of supporting the data structure.