With all the connections for the analogue sticks, the analogue output, the lcd-control, and the buttons, the 28 standard pins of the Teensy 3.1 were used up. In order to connect the XBee Pro S2B with the Teensy, I had to use the solder-pads on the bottom of the pcb. After all the work was done, I realised that using those pads for Serial2 instead of the standard pins was more difficult than anticipated. Thanks to the tips from the PJRC Teensy community forum, I managed to get the serial link working.
With all the peripherals set up, I still have to figure out how to drive the lcd, as the panel used in the mc-17 differs from the standard alpha-numeric ones.