I have had a spare Birdgestone 90 motor up on the bench, trying to unstick it. Unfortunately the piston is good and seized no matter what I soak it with, or how much brute force I use. The least i can do is grab some of the parts off it, such as this clutch mechanism.
My blue BS90 sport has just a little bit of clutch slippage at high rpm in 3rd and 4th gear, so I plan on cracking it open to inspect. Usually this means the fiber disc and metal plates just need a bit of cleaning. But in case the discs are worn, these spares can go in, as they measure out just fine in terms of thickness.