Considering they were an option from the manufacturer, I would imagine they would not require an engineers certificate but, I recommend you talk with your local RMS to confirm if legality is something you are concerned by.
To be honest, I would be very surprised if anybody except an s30 enthusiast would pick they were not the original brake setup.
On another note, I have been researching another vented rotor option whilst maintaining the original caliper.
I have found a couple vented rotors that need a small amount of machining to suit the 260z hub and have an appropriate offset to use the original calipers but with a spacer placed in between the caliper halves.
This option would strictly need engineering certification but will also look quite original so probably difficult for a non s30 enthusiast to pick it.
I expect it to be a bit cheaper and lighter than some of the other options currently used but will see how we go.