Ethernet cables

To replace X + Z ethernet cables can I use ethernet flat cables?

Replacing the X and Z ethernet cables with flat cables should be fine, however you need to make sure the specs of the cables are the same, especially the gauge of the wires within the cables.

The other thing to note is that the locations where the cables enter the machine on each end were designed for round cables, so flat cables may be harder to fit and/or seal around to reduce chip ingress.