OS 9.1 is supported on x100 series Macs so long as you use an original Mac OS 9.1 install CD. You cannot use a Mac OS 9.0 or 9.04 CD + the downloadable 9.1 updater.

If you don't have an original OS 9.1 install CD, get your hands on another PowerPC Mac with SCSI. Remove the hard drive from the PowerMac, and connect it to the other Mac, either internally or in an external enclosure, and run the 9.1 updater on that. This really does work - many years ago when my 8100 worked, I got my 8100 updated to 9.1 by putting its drive in an external enclosure and running the 9.1 updater on my PB1400.