Sources for these files can be found in:
https://github.com/rwmjones/fedora-riscv-bootstrap

To run the stage3 you need riscv-qemu from
https://copr.fedorainfracloud.org/coprs/rjones/riscv/

and then you should be able to run:

  qemu-system-riscv64 \
    -nographic \
    -machine virt \
    -m 2G \
    -kernel bbl \
    -append "console=ttyS0 ro root=/dev/vda init=/init" \
    -device virtio-blk-device,drive=hd0 \
    -drive file=stage3-disk.img,format=raw,id=hd0 \
    -device virtio-net-device,netdev=usernet \
    -netdev user,id=usernet