Hello,
It is per se possible to find some (relatively expensive) USB-to-SCSI adapters that possibly could be made to work with certain newer computers, but the driver software availability is probably going to be very limited. Also, a special Mac-disk utility would be required on a PC. It is usually much better to try to use an old Macintosh computer with SCSI instead (files could then be transferred to a modern PC or Mac through floppies, via a local network or over the Internet, depending upon model and operating system).
You mentioned that the Power Macintosh 6100 does not boot. Did the computer react at all (startup sound, hard drive activity sounds, display)? A low logic board battery voltage could cause a "no video" situation on a monitor connected to that model.
Jan