Absolutely you can - I have done this many times and then sold or given away the old drive as an internal. The only issues are a) what SCSI standard it is using (SCSI-2, Wide SCSI, UW SCSI, etc.) and b) SCSI ID selection. The replacement drive would have to match the existing drive for SCSI standard and cables or you will have to retrofit it. Most likely you'll have to select the SCSI ID manually before installing it or figure out the connections from the external selector and retrofit them to the new drive.
I thought a thought, but the thought I thought wasn't the thought I thought I had thought.