Reading through your first post again it's not a ZX-R slipper you have at all, it's the ZX ball diff. Seen the words ZXR and mentioning slipper parts in your original post and mistook it for the ZX-R.
The thrust bearing works with the centre ball diff, the layshaft has no slipper function. Try running the ball diff loose enough to act as a slipper and you will have no drive. A common mod was to take the balls out of the spur gear and tighten up the diff nut to lock up the diff.
The correct manual is here
http://www.retromodelisme.com/manuel...zx-kyosho.html
From your description you have not listed the spacer that goes inside the rear drive hub, although I presume you will have missed the spacer being a separate part as it fits in flush with the drive hub.