AFAIK, the PowerMac G5 did not ever come from Apple with a CD-RW, so the drive is not original anyway. Replace it with a good DVD burner. You would need to find a burner with PATA interface. Those PATA drives are quickly disappearing, but there's still good buys: http://stores.nextag.com/store/47980...-On-IHAP122-04
And, you can find a SATA PCI controller card to install, and you could then use the much-easier-to-find SATA burners.
Also - the G5 won't boot to a USB drive, so an external will need to be Firewire interface.