If the bike ran fine , when you got it , I would clean out the carb , and check the reeds while the carb is off.
The head gasket would then be next , unless you have any signs that its leaking.That is , loss of coolant , or pressure building up in the radiators.
The cr´s head gasket is an old fashioned steel gasket , it doesnt cost much , and its an easy job. Be sure to tighten the head to the correct torque , if you need to remove it. Maybe its a good idea to change the piston as well , if you are not sure how many hours since the last top-end ?