Replacement NGK B8ES spark plugs. This is the normal replacement plug. Fits all years of Honda N600 sedans and Z600 coupes. Replace that fouled old thing with new. It's pretty tough to drive your beloved beast when it won't start!
Note: these are non-resistive plugs and are discontinued and thus hard to get now. We also carry the resistive version, which is the superseded product (NGK BR8ES).