mate you would be exactly right to think head gasket it will def be that they are very common and i do loads of them well done for noticing the water on the plug most people would miss that ! i would reccomend only using a genuine gasket on it and change both the head gasket and water jacket gasket around the edge of the head.dont use a pattern one they turn to bubble gum after a few weeks.good luck with that im pretty sure there is a bleed screw on the top of the head after refilling the coolant be sure to bleed out the air until it runs out with coolant only.if you need help getting a genuine gasket let me know i work in a bike shop as a mechanic.:)
adam bike doctor
February 2009