Really need to know the year and engine size.
But you can check this, Does your engine have 2 lines just above the oil filter that run up the side of the engine and goes into the radiator on the drivers side ? If so then that is the engine oil cooler lines and the tubes in the radiator were the oil flows through to cool the oil are probley leaking into the radiator coolant. IT happens often.
WILL NEED TO REPLACE RADIATOR. And flush out the cooling system real GOOD.
Did your engine freeze from lack of antifreeze ?
If the radiator fluid is cheery red looking then the transmission cooler that is built into the radiator is leaking. REPLACE RADIATOR. GOOD LUCK.
Blown head gasket.
If the Chevy Celebrity radiator is smaller that the Saturn radiator it might well overheat from not enough cooling capacity. If the Chevy Celebrity radiator is the same size or larger than the Saturn radiator I would think it would not over heat.
That requires an engine scanner to set the timing on that engine. The timing is computer controled.
Radiator cap
A 3 or 4 core radiator would be your best
YES. that would be a direct bolt in/swap
The pressure from the radiator is pushing out through the overflow canister. Probably solved with new radiator cap but could be an indication of head gasket problems if pressure gets too high.
A radiator cap would not cause a leak! However, if the radiator cap is holding pressure, other weak parts could signal issues in need of attention.
I would suspect a clog in the radiator.
Low on coolant. Cooling fans are not operating. Faulty thermostat. Air flow through radiator is restricted. Coolant flow through inside of radiator is restricted. Bad head gasket.
maybe its automatic transmission fluid
radiator cap