peugout sppedfight2 will not start?

Hi I have my sons speedfight which will not start. It was working fine but then just stopped. After a while of sitting in the shed the battery went flat so i replaced this. I was then unbale to get a spark. I have changed the coil,HT leads and spark plugs. Horn lights indicators etc all work. I have also noticed that the immobiliser light does not come on at all. the cdi in there at the moment is an ac100 02. I want to eliminate all the cheaper options before i shell out on a new CDI. Any ideas.