Hmmm...ok, found a good thread on Apple Discussion Forums that has some helpful info, including specs on some fan options, as well as the Apple p/n for the original (which isn't in the service manual!):
https://discussions.apple.com/thread/1613435?start=0&tstart=0
Original fan p/n: 922-4580
Available here: http://www.dvwarehouse.com/922-4580-Apple-Processor-Fan-for-Power-Mac-G4-Quicksilver-p-36984.html
Still used though. I'm going to do some comparison-shopping at DigiKey or similar and see if I can find a new replacement that's close enough in power/noise/CFM. I'll update w/ what I find.
Thanks!
Fred