Start simple and cheap. Put a new radiator cap on it. If that doesn't work, go from there.
I had an oil leak on the inner clutch cover that caused water to pump into the oil. It was a minor thing, but I ended up doing the top end to find it. New head gaskets require a resurfacing of the heads to do it properly. It can be done without replacing rings if you are careful and leave the piston heads in the cylinders. Some mechanics just pull the cylinders off of the pistons, and then you need new rings. Replace chains, chain guides, and tensioners while you are in there. Some of your plastic chain guides will be worn.
"I'd like to meet the joker who had the nerve to call this a road!" -- Walter Sigmann
"Gravity is a very fickle mistress." -- Unknown
"Throttle to the donkey!" -- Marc Coma