Another thing to check, on that old of a model, is the resistor in the tail light assembly. If it was firing, and now just all of the sudden, you can't get it to spark, unplug the tail light assembly (after checking the bulbs, as suggested in this thread), and check for a broken lead to the resistor. Also check to see if the plastic around the area that the resistor sits has melted...if so, the resistor has gotten too hot, and is probably no good. As far as I know, you have to replace the whole plug-in section of the tail light assembly, as I don't think that the resitor by itself is available anymore.
Hope you get it back running soon!