Rename the original Mcd001.ps2 to Mcd001_original.ps2 . This ensures you have a fallback if the new save corrupts.
If you are looking for pre-completed save data to bypass early levels or unlock everything immediately, several high-quality "100% complete" files are compatible with AetherSX2 (often in .ps2 memory card format): god of war 1 save data aethersx2