Since TronXY hasn't yet released their source code for the X1 Melzi board, I attempted to build the current Repetier 0.92.9 and source all the necessary settings. Most of them are copied from the X3, which is in a way very similar.
All necessary configs for the X1 were made in the Configuration.h file.
I'm still experimenting
As always you compile the firmware with the Arduino IDE from [arduino.cc](from http://arduino.cc) (tested version 1.8.2 on Mac)
To set the right compile target add the Sanguino board definition URL
https://raw.githubusercontent.com/Lauszus/Sanguino/master/package_lauszus_sanguino_index.json
to Additional Boards Manager URLs in the Arduino preferences.
After that select Tools > Board > Board Manager, search for Sanguino and install the board definitions. (tested version 1.0.0)
Compile target settings:
- Board: Sanguino
- Processor: ATmega1284 or ATmega1284P (16Mhz)
- Port: (select the right port)
Hit compile and the Arduino IDE should hopefully compile and upload your new firmware (It may be necessary to burn the Arduino Bootloader the first time, please follow this guide )