As said, your cable system needs rebuilt. Your whole heater system may need "rebuilt". All of the parts are available in the aftermarket. I rebuilt my deluxe heater and will use a remote valve positioned right outside the fire wall and operated by the lever and cable on the dash.
Click and Click

May take a slight modification to outside box lid to fit new core.

This one has some dimensions.


Right is right, even if everyone is against it, and wrong is wrong, even if everyone is for it. - William Penn