I 'think' the type of generator fitted to these bikes may need the battery to help provide initial spark, OR, perhaps not if it is fitted with the two way switch mounted under the headlight. This if I remember correctly is similar to Brit bikes that had an emergency start function, you can switch it to the alternative position to bring in an extra part of the generator to start the bike when the battery is flat. However, if your battery is ok and the spark has gone then problem is elsewhere. Try different plug, then lead, cap, or even coil one by one to find the problem. Are connections all good at the points, and make sure that nothing is touching an earth that shouldn't. Very simple electrics, should not be too hard to isolate the issue. Whereabouts are you?