There are heaps of things it could be. To start with use new plugs check they aren't just bung. First check is your earth (multimeter from negative battery to engine - should be 0.x ohms) next your stop switch, disconnect the kill if possible (dunno on your ski but 701's have a black plug you can disconnect) then into the electrical box onto stator, earth to lighting and earth to ignition (one will be a about 10-20 ohms the other about 300-600ohms), then common should have low ohms, coils one side will be a few ohms other should be a lot higher e.g. ~4kohm, , check the leads aren't broken by pulling the wires with pliers, check the resistance across the caps. Last check your cdi - usually need to swap it out... Then you are into the harder to find problems, wiring, etc.
