:welcome:
pardon my ignorace Giorgio hello.
Now what exactly is happening with your mower when you try to start it ?
What tools do you have to work with ?
Measure the voltage between the white wire and the - terminal on the battery , switch off, switch on on and switch in start position.
Do the same between the + terminal on the battery and the white wire.
This will tell us if it is a kill wire or a start wire and from that we can work out where it should go.