Guys I am in QLD so tell me to be quiet, but an engineers cert is inexpensive here, and that is if the girls at transport even notice. I only paid $100 for my last one and that included brakes, engine, diff, gearbox and suspension. (R33)
The ADR are available from COMLAW.ORG however they do not address what you are talking about, the national code of practice covers vehicle modifications in QLD and I assume that national means that, national. It is QLD though, we have strange law makers. I have both on disc, PM for offsite backup.
Where you come unstuck is when the engineer makes you comply with the emmision control for the vehicle the engine is sourced from, when going to newer model engine. As I type this I wonder, what emmision controls are there on the patrol? Being commercial type I would expect much less.