I can see immediately that the Zenith is not compatible inside that case: the Ctrl keys are 1.75, not 1.5 and the spacing between key numeric row and function key row is too large.
Even if you do find a replacement keyboard that has the right layout, it could be too large on the inside, so
do google for images of how the innards look like!
If you do know some rudimentary C programming and can solder, you could go with what you have got. I would assume that the two ribbon cables are connected directly to the matrix of key switches - where each key switch consists of a pair of conductive pads on a membrane.
One cable would be the (logical) columns and the other is (logical) rows -- which is which does not really matter unless it is a fancy keyboard with a diode at each switch (but this is Amstrad and they never made any fancy stuff that I can recall). Each switch is on the intersection between a row and column, connecting one to the other when pressed.
The layout of a keyboard matrix almost never corresponds very well with the actual layout of the keys - you will have to find it in a manual, from someone that has already reverse-engineered it (if someone has), or reverse-engineer it yourself using a multimeter.
First, count the number of leads. You will need to either connect all of them to GPIO pins, or use a multiplexer of some kind for (what you choose for) the columns as only one needs to be strobed at a time.
Is there any keyboard software for the Raspberry Pi and the OS you are planning to use?
Most DIY:ers here use a Teensy 2.0 (25 GPIO pins) or
Teensy++ 2.0 (more pins) microcontroller board and the
TMK keyboard firmware with a specialisation for the keyboard matrix.
