If you do an image search for Baby Nambu holster, you will find that it is the same "clamshell" type as the full size Nambu.
The only way to return it to the original 8mm Nambu would be to buy an over sized barrel blank, chamber it for 8mm Nambu, thread it for whatever threads are on the slide, lathe it to the proper contour, and add a front sight. You'd probably spend more than you could pick an authentic example on Gunbroaker.