My son and I rebuilt the ball joints on his 86 951. We used the Rennbay kit with bronze bushings.
Porsche 944 Solid Bushing Ball Joint Rebuild Kit
We'd had the old kit with delrin bushings installed a couple years ago, and the delrin bushings failed miserably - which I suppose is the reason no one offers that kit any more. On both sides, the bushings deformed and basically got spit out of the pockets.
Hopefully, the new bronze bushings will last.