Your Prelude is actually a 2.3 liter engine coded H23. Your best bet is to replace your engine with a low mileage used Japanese H23 engine. These engines have really low miles on them and it will probably cost you about the same- except now your engine will have less miles on it. It sounds like your head gasket. The scenario that I'm thinking of is that the car had no coolant- which is why it overheated in the first place. That blew your headgasket-which allowed oil into your cooling system. However , since there's no coolant to mix with the oil in your radiator, all you see is oil.