If you already use newer Korg software or hardware, you can download official recreation packs:
This is a high-quality emulations of the Kawai K1, a contemporary of the M1. It captures the grainy, lo-fi digital charm of late-80s hardware perfectly.
or free community alternatives that capture its iconic 90s sound. 🎹 Official KORG M1 V2 (Free Demo) The official KORG Collection M1 V2
You cannot legally get the exact Korg M1 plugin for free on Windows/Mac desktop in an unlimited, official capacity anymore.
The "M1 sound" is currently seeing a massive resurgence in Lo-fi, Synthwave, and Deep House. Using these free tools allows you to tap into a sense of nostalgia while providing a thin, punchy frequency profile that sits easily in a modern mix without overwhelming your CPU. Conclusion
While not free (the full version), Sound on Sound magazine (October 2004 / June 2005) wrote a definitive technical review of the M1 plugin.