In the cam.wdl is where the camera panning is handled:

Code:

---

	if(mouse_right == 1 || key_pressed(tl_key_scancode) || key_pressed(tr_key_scancode) || key_pressed(ldown_key_scancode) || key_pressed(lup_key_scancode))
	{
	    orbit_camera_pan -= (mouse_force.x * mouse_sensitivity * mouse_invert_pan) - 
	                    ((key_pressed(tr_key_scancode) - key_pressed(tl_key_scancode)) * mouse_sensitivity * mouse_invert_pan * time);
	    orbit_camera_pan = orbit_camera_pan%360;	//limit to 360 degrees

	    orbit_camera_tilt -= (mouse_force.y * mouse_sensitivity * mouse_invert_tilt) - 
	                     ((key_pressed(ldown_key_scancode) - key_pressed(lup_key_scancode)) * mouse_sensitivity * mouse_invert_tilt * time);
            orbit_camera_tilt = clamp(orbit_camera_tilt, orbit_min_tilt, orbit_max_tilt); //limit
        }

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There are some checks in other parts of the system that check whether mouse_right and mouse_left are pressed or not. You'll have to change those if you change the above mouse_right to mouse_left. I'm guessing you'll have to switch all the rights to lefts and vice versa.