Replacement NGK B8ES spark plugs. 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: BR8ES, which are more readily available, are a resistive plug and has 5k ohms in series, and supersedes the non-resistive plug. Although they are rated to be functionally equivalent, resistive plugs do raise the voltages in the coil and wires, which can potentially cause unwanted results and performance issues. The reason for resistive plugs is that they reduce RFI (radio) interference.