Help to repair mbk ovetto moped (2004)?

was running quite happily then just decided to slow down and stop and wont start again, we have checked the spark plug, (its firing ok) petrol is getting to the places it should be getting to. When we took the spark plug out there was a small amount of oil around the outide of it, ( none on the tip), does anyone know what might be causing it not to start???

tanya
December 2007