Do you, know, if by any chance the decompilation happened in Ghidra and if so, if the decompiled project is accessible somewhere? There is Abbree AR-2520 which has GPS, 128kB/32kB CPU and up to 25W, and whilst it will never have DMR and reach the potential of OpenGD77, it uses same codebase as the RT-890 but on a different processor (most likely a Chinese clone of AT32F415CBT7) so OEFW with some modifications would be possible. If the functions could be mapped from RT-890 the reversing would go much faster...
73, OK2MOP