well, first throw it away then buy a new one.
buy a new engine. your head gasket has gone bad, a rebuild is not a good idea because your heads could be warped. run it till it blows up, buy used motor.
the car over heats which can cause the heads to become warped or even cracked fix it asap that's something that you cant hold out on it will cost alot to fix it if you do ....
Have a compression test run to see if you have a problem with the head or gasket. Once you find out you do, then you must remove the heads in order to determine what the problem is. Have the heads inspected at a machine shop to determine if they are warped or cracked. Once the heads are removed the gasket must be replaced so it really makes no difference.
you cant
replace it
Cracked or warped heads, motor block, Bent valves, blown head and/or header gaskets worst case locking up the engine.
Could be many things like a stuck thermostat, blown head gasket, warped heads, bad water pump etc.
Depending on the engine ball park $700.00 to $1500.00 or higher if the heads are warped etc. There are two heads, never replace just one gasket.
A couple of things- Does engine start well, cold or hot? Have you noticed loss of power? Does it smoke?(maybe an indication of a blown head gasket) Do you notice water in the oil?(another indication of head gasket failure or warped heads) Do you notice an engine miss (a putt-putting sound at the exhaust pipe) There are other indicators as well, but these are the most common with an overheat-damaged engine. I just put a 3.1 in my son's car and the reason was that the heads warped so badly that it was cheaper to find a good, decent mileage engine.
You might have the heads reworked, probably an air leak if it's over heated before and warped the heads. Very common in aluminum heads.
Actual engine oil in the radiator would indicate a blown head gasket or warped engine head. There are just too few places that the engine oil and coolant find close proximity, and the heads are the most common place to introduce this mixture.
blown Head gasket/warped heads/cracked block