You have to run the Item Organization assignment process. Usually,we develop a script to perform the Item assignments in implementations.
Will check Item Organization assignment Process.
Currently we do not have any script to do it.
Will need to check.
Also, you can use 'Item Import' process.
Just insert all the items in the master organizations into MTL_SYSTEM_ITEMS_INTERFACE with new organization_id and run 'Item Import'. Try this for any one item first and proceed if successful.
Please refer to document Understanding Item Import and Debugging Problems with Item Import(Doc ID 268968.1)