I've had this issue before too, after some research I'd suggest downloading the .gci to your computer and transplanting it into an emulator's memory card tool. If the problem really is a corrupted memory card then this is the only reliable method of getting what you want without tinkering with the broken one. Also, I went looking for Gamecube cards recently and the highest price I saw on them was $15, so they aren't too bad if you choose to go that route.