check the inside of the header tank for milky / scummy residue , if so might be the head gasket , if not could be valve stem oil seals or piston ring-s gone

piston rings can be checked with a comression tester on each cylinder, quite easly done but means a rebuild if there gone

and valve stem oil seals can be checked by whiping the cam cover off , if there still soft n maluable then there fine if there hard n brittel then they idealy need changeing and normaly means a head rebuild

some one else must have another few ideas