NGK DPR8EA-9 or ND (NipponDenso) X24EPR-U9. For Iridium, there is the NGK DPR8EIX-9. You can also use a Champion 809(RA6HC), which is available with a 5/8" hex so you can use commonly available spark plug sockets. The Autolite 4163 and MP4163 (premium) are also choices. Gap is 0.032-0.036" (0.8-0.9 mm), torque to 10 ft-lbs (14 N-m).
NGK, Stock- DPR8EA-9, Platinum- DPR8EVX-9, NAPA Stock # 2872, recommended gap- .035 inches, Iridium- DPR8EIX-9, NAPA Stock # 2202, or, DPR9EIX-9, prices seem to range from about $6 to $9 dollars each.
DENSO, Stock- X24EPR-U9, Iridium- IX24B.
AUTOLITE, Stock- 4163, Platinum- MP4163.
ACCEL, Stock-0882.
CHAMPION, Stock- RA6HC, Alternate Premium- 8809.
MOTORCRAFT, Stock- HGS22C.
AC/DELCO, R121XLS
SPLITFIRE, Stock- SF-416C, Platinum- TP-416C.
BOSH XR5DC, This is a resistor plug (as it should be) with a hardened silver electrode. Taking them out of the box, you must gap them to Kawasaki specifications. You may have to order this plug through a foreign car parts store that carries the full line or has access to the BOSCH motorcycle line of plugs.- Thanks to Scott Borderud for this info!
This is the info I found so a Champion 8809 should be Platinum and Free and work fine!