After installing the InstantCake image, you would need to run a C&DE, which means the unit would have to be run through guided setup to be able to record. If you are going to hack, fakecall.tcl, followed by a reboot, will work effectively.
Another alternative would be to make an image from the existing drive, if it isn't too far gone, and then restoring the image to a new drive. If you follow that route, you will not need to run through guided setup, as the image will match your receiver. Instructions are here.