Just a guess but somewhere between 5/32 and 5/15th will be close enough to get it running. Big problem with Villiers was the were using the (prince of darkness) Lucas for their magneto coils and they were pretty terrible. I have fitted an external coil on a couple of small 2 stroke motorbike engines and small battery to provide spark. They worked well and were easy to start but this is probably not going to help you.
