You are close there are a few things you need to do, and one of them is create a whole new furnace
When you create a new item, you are actually creating a new recipe and not an item, the item is just a biproduct of the recipe, those new recipes cannot be made in vanilla structures, so if you wanted to tweak the forge you actually need to;
- Create a new item|recipe and make it a child of what you want to duplicate IMPORTANT
- Create a new forge, item, structure, engram
- Remap the old forge to your new forge in the engrams
- Remap all the old recipes, crafting requires inventory (gasoline is the only other forge one I believe at this point)
- Create a new inventory & remap default items, remote add only items & default engrams
- Remove all forges from the world, warn players on your server not to use the blueprints they find in the world, or go through and change all loot caches too (while your at it, stop level 60 drops from linking to level 25+35)
Warning If you did not make your new metal a direct child of the old metal, you need to recreate every single recipe, engram and structure in the game that uses metal, including the drops from dinos and loot cache