FIFO Costing of WIP Components for Purchased item

I have a business scenario that I'd appreciate some help with. The company is using FIFO costing and is using R12.1.3. They buy items that can be used as they are or can be disassembled to the component parts and the parts are used. I can use the WIP Work Orderless Completions functionality to convert the purchased item into its component parts but I also need to allocate portions of cost of the purchased item to the component parts based on a %age value. Is there any way to do this without customising the Layer Cost Worker? Logically it is the equivalent