updated: 01/21/2020 Since the release of the MMDVM in early 2016, Johnathan Naylor’s (G4KLX) creation has hit the ground running throughout the ham community. This simple creation has put the power back into the hands of radio operators around the world allowing them to build and tinker with...
I'm currently trying to do the same thing. I just want to make a portable DMR and P25 standalone repeater out of two CDM 1250s. I haven't been able to find a lot of info about this. If the OP got this working let me know.
Scott Zimmerman of Repeater Builder makes a nice dual slot Pi-Star (that uses MMDVM) that you can easily wire up as a standalone DMR, DStar or Fusion repeater. (I have version 3 of the "Complete Raspberry Pi3 / STM32_DVM Combo".) You can connect it to an MMDVM server, like Brandmeister or TGIF, anytime you want over the Internet, but it's not necessary for a local-only repeater.