es, you'll need to find and buy or burn a Mac OS 8.6 CD that originally came with that computer, boot from it, repartition+reformat the drive, and then you'll be able to restore or reinstall that OS version.
eBay might have, it Mac Garden appears to have a copy, although I haven't used this image before: https://macintoshgarden.org/apps/mac-os-85-851-update (Look for PowerMacG4CD_86.ZIP on that page, download number 18.)
@Gorgonops have you tried the tap interface for QEMU? I've got a lovely install I just did with slirp networking in os 9.2.2 but can't access my a2server Appletalk server. Doh! That's kinda how I was hoping to get software onto the fresh install. FTP works still but Appletalk would be easier.