CASA however still require the full deal including engineering orders for the installation.
Say you have a 172 with a 430W and KT76A already fitted, surely you can just fit a KT74 that slots straight into the rack using the FAA AML STC.
FAA STCs are issued by an acceptable foreign authority so should suffice in that case, then it's just labour to run the GPS data wire to the rack and set up the new unit.