The original bootloader is still on the machine and I will explain how to use it. The 1.99 .syx file is included with all 2.x firmware releases.
- Attach the Mono to your computer with a USB-B cable, hold [PATTERN] and [RUN] (this is the original bootloader) and turn the Mono on. All the sequencer lights will flash in unison indicating it is ready for update.
- On your computer, open a program to send sysex (SysEx Librarian on macOS, for example).
- Make sure in the settings to set the transfer delay (between messages) to 1 or more.
- Send the 1.99.syx file to the Mono. The sequencer lights will turn off and then turn on one at a time. When the transfer is complete, it takes a few seconds until the lights turn off.
- Turn Mono off, and then start the purge of memory. You do this by holding down [FOLLOW], [F#],[G#], and [A#} simultaneously while turning Mono on. The sequencer lights will do the one-at-a-time turning on several more times, at first slowly, then faster. It takes a while, so be patient. Eventually the sequencer lights turn off and the keyboard lights turn on. At this point Mono should run nicely with the 1.99 firmware (which I believe is the same as 1.2.3 except that a program dump bug was fixed which would have prevented the loading of the new bootloader and the 2.x firmware).
I hope this all goes smoothly for you!