So you want to find +damage from items? You could just check for all +damage item abilities on the unit, and add all of them up. You'd have to check for all possible +damage abilities they might have, but it wouldn't be too bad. If that's not what you want please clarify.
You need to be able to get the damage each item gives, then u can just go through all items in inventory and add all damage.
Or you can trigger the damage the item gives.
I believe that would only add up default item abilities, and bonuses added using his mod. So if you use purely his mod, then yes it would be easier. But I think Stock probably has most or all of his items made and wants to find their damage. Dunno though, depends on what he's doing.