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
Use https://platformio.org to compile.