You'll need a scsi host adaptor card, like a C=A2091, Dataflyer, GVP HD+8, Supra, ICD or similar. Of course you'll need a suitable 50-pin scsi-1 or scsi-2 drive (any speed will do). After you've installed the hardware and properly configured the scsi chain and termination, you'll need a cdrom driver package to use the drive, either OS3.1's cd0: devicedriver or better yet AmiCDFS or AsimCDFS or the like. Enter the proper device driver (scsi.device, gvpscsi.device, dataflyer, etc.) the scsi ID number of the drive and it should work.
For rtg on an A2000, look for a Zorro2 compatible card (Picasso II/II+/IV, Cybervision64/3D, GVP Spectrum 28/24, Piccolo, etc.).