Like the man (the EBay man) says, you can't fix a slightly warped or grooved sohc4 disc unless you've got a surface grinder. Sandpaper is fine for "deglazing" your discs, but that's about it.
Due to the heat generated by spirited braking from speed, the discs (already very bloody hard stainless steel) become
case hardened stainless steel, and are just about impossible to machine with cutting tools on a lathe or mill. I know, I've tried............
Looking at the EBay pic of the rotary surface grinder though, I've just realized that provided you could buy a suitable stone, it wouldn't be too difficult to grind your own discs on a mill/drill with a rotary table, or with a little "home grown ingenuity", even a cheap pedestal drill.
Main thing would be to use plenty of coolant to keep the stone cool, and sharp. Hmmmnnn, I might give it a try on the weekend!