Most motorcycle disc rotors are stainless steel with a higher steel content because it's still magnetic.
Cast iron is to brittle, specialy for off road bikes. The only bikes I can remember that had cast iron discs where old Moto Guzzi's.
I drilled holes in the discs on my '98 315R and I can tell you that they where much harder than ordinary steel: I used a carbide drill for hardened steel which was blunt in no time.
