I'm sorry to say that the equipment you'd need to fix that scratch would probably cost a 1000 times more than the drum.
The green stuff is a photoelectric sensitive material that is evaporated onto the drum under vacuum (afaIk).
In use in the copier it is first charged with static electricity, then it's illuminated by the image which makes it conduct and dissipate the static charge at the bright areas. Then it's passed over the toner (a plastic powder) which is drawn over to the drum at the dark (still static charged) areas. Then the paper is passed over the drum with a stronger static charge on the other side which makes the toner hop over onto the paper. Finally the toner is fused to the paper with hot silicone rollers.
I don't know about any selenium-sulphide paint, so the only real option for you apart from replacing the drum is to fill the scratch with an insulating varnish, which would make the scratch appear white on the paper instead of black.