You can change
which uses pin 2, to
which is pin 3 on the Uno. These are the numbers of the external interrupt. The Uno only has two of them, which are tied to those two pins. So as long as you are using the Arduino Uno, you need to use one of these. The Arduino Mega for example has more external ...
Modifying/Making a variant
If you wanted to do it in an Arduino-like way it would require editing the Arduino Due variant.cpp or copying the making your own variant, say from a copy of the Due, and editing the extern const PinDescription g_APinDescription array to include an entry that matches for PC11.
As an experiment I made the following ...
(Moved to "answer" for space; it's definitely not an answer. Sorry all!)
It's a motor controller shield; the GRBL is on the Arduino.
You'll need a motor controller at each motor; not clear if that's understood.
I'd also consider optical comms, although it has a different set of problems.
Ultimately I might approach this in a ...