I run OpenRTX on a Retevis RT3s, which can be done without any hardware modification. (I do not know if original firmware is available somewhere -I have not checked-. If that is the case, it should be possible to reflash the stock firmware on the device).

Anycase, I must say that M17 does not run correctly on that radio. There seems to be an issue that the first 300 ms or so of the transmission is not correctly modulated (something related to the FM modulator) and also the end of the transmission is broken of halfway the end-of-transmission frame.

I am currenly at the stage of trying to understand how OpenRTX really works, and my first idea is to implement POCSAG-paging into it. (As I have the source-code for that here anyway) and I also have some ideas for APRS to want to delve in.

(OK, that is, if I have some time left next to all the other stuff I am working on :-) ).

73 kristoff - ON1ARF