Did you add the P1 upgrade board or are you using the PMU + GCU? If you used the upgrade board it should have come with the tilt lever for the controller. Here's the part by itself, but I'm not sure if it will work with the PMU + GCU setup: http://www.dronefly.com/product-p/lever.htm
Yes, it's the V3 upgrade board, and it does come with the lever (made of some polycarbonate plastic).
I didn't know if I need to upgrade the internals (other than adding the lever on the back).
That's it No other hardware modifications necessary. However, after installing it you will want to move that lever back and forth while performing a regular controller calibration, and then calibrate the X1 channel in the NAZA Assistant after that.