I have NEVER seen a 486 with USB ports.
first generation USB happened before ISA went completely out of fashion, so ISA USB cards may exist in such places as Oren's barn. (i think)
Thus drive removal is a viable method. And pinout not being mentioned aside- there are adapting plugs/cables out there and should be trivial to find.
we still have the "but then what" question, which is, what FS does SCO use, and what partitioning scheme, and so on. If you attach the drive in question to the SCSI bus of a machine with a knoppix 4.0 CD in the cupholder and power cycle, will it automount? Alternately, if you attach a SCSI CD reader to this machine, can you boot from it? I think I have one of those sitting around gathering dust at this time.