It sounds like the water pump is bad. The only other reason I can think of is if the block is cracked but unless it's been very cold and you don't have the proper amount of anti-freeze in it that is not very likely.
I had a similar problem and ended up replacing both the lines and the radiator.
Sometomes it is hard to get all of the oil out of the cooling system. However the place that replaced the engine should flush the cooling system until it is clean. Also if your car has an engine oil cooler , it could be the cooler in the radiator and not have been the engine in the first place.
Could be a small crack in the plastic radiator ends, It happened to me, you do not see the leak but when running the water escapes. It could also be a crack in the head (very bad)
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.
Saturn radiators don't have caps to check and add fluid to the radiator you open the overflow tank and add it there
Stop leak is a temporary fix. I don't recommend it. Tighten the clamps on the hoses going into the raditor.
It's an aluminum radiator with plastic tanks.
It's only radiator cap is on the coolant reservoir.
The location will more than likely depend on the model of Saturn
radiators are in the front
On the 4 cylinder Saturn, follow the top radiator hose back to the engine. Note that there is a housing that is bolted to the engine to which the radiator hose is connected. Remove the radiator hose first, then remove the housing. On the 4 cylinder Saturn's, the thermostat is a part of that housing.
A 1994 Saturn SC may not even have a radiator drainplug. In order to find out if the 94 Saturn does have a radiator drainplug, a person can take off the lower hose to see, or look in the car's manuel.