We have had this discussion before. Here is the Official Definitions of the Approved and Authorized terminology:
Stock: A bike with factory frame, factory engine, etc. regardless of how much bolt on shine there is (regardless of which factory. Production is production)
Custom: One of a kind
So, you can have custom handlebars, paint, seat, wheels, lights, trim, fenders, but have stock engine and frame... it's stock. Or if you prefer it's "customized" or "personalized", but still stock. And by the way, most Stock bikes are way better to own and ride than 99% of the customs!