Are the non-inventory items unique ie only ever Qty 1 on hand?
What is it about the item that you use to identify it as relating to a particular motor from a particular customer?
On the main item, are you using Cases or Job Numbering that maybe can also be part of this 'inventory'.
It could be a simple Custom Record with a unique name like {Customer] - {Item] - [Date] and on that record you have a 'Status' field like 'Received to Workshop', 'Awaiting Inspection', 'For Machining', 'In Machine Shop', 'Returned to Workshop'.
I have a use case where a customer receives back Hire equipment. Before some of that equipment can be re-hired it has to be serviced/checked. It's company assets which we process with a Dropdown (controlled by Workflow buttons).
How about a connection to the Sales Order/Work Order which cold record movement.
In both these scenarios, a saved search of the movement status would be the listing.