The 1.8 Toshiba hard drives used in the iPod are not available for purchase. They used to be sold only to Apple. Now there is another product using them, but the Apple ones still have special firmware and a Apple "tattoo" (silkscreen) on the drive indicating so. Without that it won't work.
Second reason you can't.. The cases are all different sizes depending on what drive and what edition of the iPod you have. In other words the case wouldn't fit the new drive.
Forget about it.. Apple is using state of the art drives.