You do not need a wiring harness if you have a 98 civic.

If you currently have a perfect 98 civic wiring harness in your car for your stock motor etc... You will have to rewire your Idle Air Control Valve plug from a 3 plug wire plug to a 2 plug wire plug & you will have to repin your IACV on your ecu from
a14 to a13. A diagram & info is avail on http://phearable.net

You might have to wire vtec sol/vtec oil pressure/knock etc... into your wiring harness if your vehicle is a nonvtec civic & you decide to go w/ a vtec swap.
You also might have to wire Secondary Butterfly Control into your harness if you go with a DOHC gsr swap.

The list goes on, just do research.

Make sure you order a 96-98 swap so you recieve a obd2a ecu so it plugs into your car without a conversion harness.

If you order a 99-00 swap you will have to get a obd2a to obd2b conversion harness & you will probably throw codes because you are lacking obd2b sensors.

If you order a 94-95 swap, you will have to convert your distributor, alternator & injector plugs on your wiring harness to obd1 via using those plugs off an obd1 harness & installing them into your obd2 harness. Fun stuff but it can be done with minor skills. You will also need a obd2a to obd1 conversion harness if you go this route.

Also, you will need
dohc radiator hoses for a civic (99-00 civic si radiator hoses)
99-00 civic rear L motor mount bracket
94-95 DOHC vtec delsol ac bracket to retain your ac compressor.