If everything was working ok before replacing the switchgear, then go back to the start. Get a magnified screenshot of the wiring diagram for inside the headlight shell, get to know what colours all the little codes mean, a cup of coffee on the workbench, a stool in front of the wheel, and carefully inspect EVERY wire and connection inside the headlamp shell. For the starter to work it needs the neutral light on, or, the clutch switch activated. The fact that you have said that your rear light isn't working casts doubt that every male connector is having a suitable relationship with a female connector of the same colour! I encountered a problem just a couple of weeks ago with the idiot lights playing silly buggers, it transpired that one of the green earth wires in the multi connector needed cleaning. Time and patience my friend. Be lucky.