1) Back up your MacMini to an external FireWire drive.
2) If you don't have another Mac, either invite a friend with a Mac laptop over, or take your MacMini, keyboard and mouse to a friend with a Mac.
3) Start up your MacMini with the T key held down, give it time to boot and then connect it with a FireWire cable to a regularly booted Mac. Your MacMini should mount on the other Mac like an external hard drive.
4) Use DiskUtility to ERASE the hard drive on your MacMini, and BE SURE TO CHOOSE: "Install Mac OS6 9 Drivers. Then REFORMAT the hard drive in your MacMini into at least 2 partitions. You may adjust the size so that one of them is large enough to restore the original data.
5) Copy a OS 9 System Folder with MODIFIED "Mac OS ROM" file. A Finder copy will work. (just drag it to the smaller partition. No need to use copy or restore utilities. Make sure the folder is "blessed" by dragging Finder to Desktop and dropping it over the System Folder icon. Or run DiskWarrior on the OS 9 partition from the booted computer.
6) Plug the external drive with the backup of your MacMini when you started. Use SuperDuper or CarbonCopyCloner to restore to larger partition.
7) Eject the 2 MacMini partitions and disconnect the FireWire cable between the 2 computers and hold the power key on the MacMini 5 secs to turn it off.
Boot the MacMini with Monitor, Keyboard and Mouse connected and hold the Option key down. PRAY to Steve Jobs who art in Heaven. Select OS 9. If OS 9 does NOT appear, boot into OS X, and go Startup Disk Pref Pane, and see if OS 9 appears there.
9) No matter what happens post an insult telling me my instructions are stupid, and you will get better, simpler and easier to follow answers than mine.