How calculate cost of an item which is part of assembled purchased product

Hi All,
We are on R12.2.10, using Inventory, Purchasing, Costing. We are using average costing.
BOM is not implemented.

We have a scenario where we purchase a top level item(Top Item) from a vendor and receive it against the PO.
This purchased item is an assembled product and has 5 different items/parts.
Item1, Item2, Item3, Item4, Item5
Those individual 5 items are also purchased as parts.

Now for maintenance/repair we need to replace "Item1" from the "Top Item" with a spare item from inventory.
After the proper maintenance of "Item1" which was taken out from "Top Item" we need to place it in Inventory. But the cost is zero. This is causing the average of those individual items to be wrong.