This is the reply I got from vicroads when asking about a ca18det into a ca18de silvia for my gf
Change of Engine
1. Replacement engine is identical to the original engine, or is an
option allowed by the manufacturer for the same model vehicle
The optional engine must be offered in Australia by the vehicle
manufacturer as an option for that vehicle (or certified variants). The
change of engine may be recorded by VicRoads provided that the
registered operator or a motor mechanic, or dealer in that make of
vehicle or an Automotive Engineer signs a declaration certifying the
engine is optional for the vehicle model, and that all modifications
made by the manufacturer for the same vehicle model with that optional
engine, have been completed.
But then as a silvia was never offered in australia so...
But sounds as though an engineer at least has to look at it