there is an ABS Sensor from Volvo 740 which works with the early Porsche 928 ABS System in S2/S3.
The part has Volvo number 3515092, and is available in the online spare part shops for approximately 30-40 euro.
The cable obviously needs to be shortened and the connection converted to fit the Porsche 928.
Short instruction how to adapt the Volvo sensor: ABS rear sensor 84_85.pdf
Andreas (and Theo)