LML 125 star- Stator issues

Hi , apologies this isn't strictly a Vespa question but my son has a LML Star 125 scooter( px clone) and he is currently on his third stator as the ignition coil keeps failing. Symptoms are it gives up after a long run and will not spark. I have replaced the Stator each time and it starts first kick and will run for a few hundred miles before dying again. I have replaced the external coil and rectifier for good measure. I have been told it may be a bad earth causing the system to overload. My question is where are the best places to check for this.  

The first stator had actually lost a rivet holding the ignition coil and the hole had become oval - possibly through vibration ?    

  He is currently awaiting recovery home on the M25 motorway on his way back from a rally so it would be nice to crack on with it and soon as he gets back. Thank you in advance