This is a frequent problem with 915s. I just finished a customer gearbox that came in for a Guard LSD. Unfortunately some of the magnets had come loose and done some damage. I reglue / reinforce with an epoxy.
My customer has his old disc, and would probably sell it to you for its magnets. Shoot me email if interested and I'll ask him. BTW, the disc is now available from Porsche Classic.
