The head gasket is not the only place that could possibly be getting oil in the coolant. You said you replaced the radiator, but it is possible that the transmission cooler is leaking.
Has there been any evidence of coolant in the oil? This would be a more common head problem than oil in the coolant. I may be wrong, but I find it hard to imagine how oil would make it into the coolant without the reverse occurring.
If the car had not had coolant replacement periodically, a corrosion problem could possibly be the problem. Even if corrosion is the culprit, the cylinder head can be welded up (after removal of course) instead of purchasing a new head. There are machine shops that specialize in welding cracks and corroded areas. It would be a rare instance where the entire head must be replaced, maybe if there was catastrophic damage such as a broken valve or piston, it might become necessary.