You may use code of motor G4GC and volume figure of 2.0 or just tell the 2004 year of production to buy liquid. There is no chance 2001 Elantra engine will work efficiently with bad cooling. It’s important to use 6 liters of coolant which has Red color. There are some conditions in servicing cooling module at Hyundai Elantra. Make sure you service system at Hyundai with installed 1.6 engine correctly.
A lot of G4GC motors are used long in Hyundai cars but only with good service. Providing you keep the rules 1.6 motor installed in Elantra of 2001 year will be long lasting. If you choose other than Red coolant or use less than 5.5 liters, you can’t expect normal work. Look at Hyundai’s requirements and make your 2001 Elantra serve longer and with no problem.