EP1/EP2 compatible receivers can be used as internal TX modules(#1457).
There is an RF port connector on the MT12 main board, so just connect it here.
It's easy to do. However, it is not possible to flash the firmware on the radio, so it must be removed when flashing.

1. Use EP1/EP2 compatible RX as internal TX module
2. Use WEB Flasher to UART flash "Generic ESP8285 Full-duplex 2.4GHz RX as TX" to RX
3. Connect RX to the RF port of MT12

RX_____RF port(MT12)
GND ---- - (minus)
+5v ---- +(plus)
TX ------ RX
RX ------ TX

A genuine internal ELRS module costs $55, but this method costs about $15 ($0 if you have a spare receiver).