If the crankcase's back has several bolts securing the cylinder head, it's completely removable, just remove all bolts, with a rubber mallet hit upper cyl head. Most times stays near solded between them and needs some vigorous hits to become dislodged.
Once cyl head is removed rub mechanically with a small screwdriver, Chineese wooden stick all crankcase and head waterways of salt layers, salt crusts, whatever is finally found obstructing inside water flow. Will ned a new head gasket, torque to specs when assembling back.
Don't need a manual, just take pictures while disassembling parts, once done with work, put them back together.