I've heard that you can make a new cable from a lawn mower throttle cable.
But yes, bikes with positron were generally bottom of the barrel and these deraillers are not collectable. Usually they came with the front freewheel system which was also not well thought of.
That said, my next door neighbor has a Panasonic with both and loves them and has no interest in my helping him upgrade. He likes it the way it is, Dammit.