Another way to do this is to use a 2-56 rod from the throttle arm to the servo. Solder a clevis to one end, and bend the rod back on itself in a small half loop (the end with the clevis). Slide the rod through the tube and connect the clevis to the throttle arm (it will come in from the front of the arm), and use a quick link from the rod to the servo arm. You might not be able to use as sharp a bend in the guide tube as you would with the flex cable. The bellcrank idea is good, too, you can drill and tap the engine mount arm to mount the bellcrank to align it with the throttle arm. Both methods just redirect the direction the control rod connects to the throttle arm.