You could make an in-line tester that will work well enough for this job Paul, they are very simple. Here is one that will give you the idea:
http://www.ebay.com.au/itm/RyTool-I...arts_Accessories&hash=item2a47e91f54All you need is a non-conducting body, with a screw in each end, and a hole in the side of the body so you can see the spark between the two screws. One of the screws needs to have its head filed down so the spark plug cap will fit over it, and the other one needs to have a strong clamp that can be connected to the spark plug without falling off when the engine runs. You screw the two screws toward each other until the gap between them in the center of the body is about 0.060". Then start the engine and watch that gap. If the sparks across it stop when the engine stops firing, the fault is in the ignition system.
If that isn't clear, please tell me and I'll get into more detail.
