the problem with most combination boxes is that the relais contact for the starter motor is burnt.
I designed my own electronics using semiconductors and I also included diagostics LEDs and error codes in the pre glow light when safety switches are faulty. You can also start it with faulty switches. Details are here: Combination box PCB , Combination box replacement and Combination box modifications
This is working in my tractor flawlessly since January now.
If you want I can repair your box, only problem is shipping time and cost. If you can remove the old electronics yourself I could also just send you the electronics board. You would have to solder the connector with cables onto the board and fill everything with potting compound.