Standard heat range Spark Plug (per Clymer manual): NGK DPR8EA-9; gap 0.7-0.8mm
Update: NGK has changed their numbering system apparently. The standard plug that we use now has the number 4929.
Alternate Replacement Plugs Are:
http://gs500.specialgreen.com/part-numbers/sparkplugs.html
Links:
Pablo's Page on Spark Plugs (http://pantablo500.tripod.com/pablosgs500page/id34.html)