Replacement NGK BR8ES spark plugs. This is the a nonstandard resistive 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: this is 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. The reason for resistive plus is that they reduce RFI (radio) interference.