I didn’t research it that much, but a good place to start would be AirMessage [1] or Mautrix [2]. Both of these require a Mac to work – it might work on a Hackintosh, though, or maybe using same tricks those forks of OpenHaystack use to run without a Mac (no pointers here, sorry). Hope this helps!

[1]: https://github.com/airmessage/airmessage-server

[2]: https://github.com/mautrix/imessage