The essential elements required for an approach system are much more than the auto flight computer, sensors, etc. I recall that there are electrical distribution differences between 73 and NG aircraft. The changes were essential to get approval for a fail operational system.
Also beware the dual Rad Alt failure – revert to Cat 1. Some aircraft use Rad Alt to schedule gain changes in the FD as well as the autopilot, thus although the aircraft is Cat 1 capable the FD performance may not be very good.