If you've got an old 6v ignition system try a BMW 6v coil and the cheapest condensor in Halfords for some kind of French car but I can't remember what. The condensor has bigger capacity than most standard bike ones and when used with the BMW coil you will have great spark. 12v coils also work but I discovered the BMW 6v one will throw a spark at as little as 2.5 volts so it's great for weak batteries or for getting you home hotwired when your charging system packs up somewhere on the road. Most 12v ones seem to stop sparking at around 8v.