first, has to be BLUE SLIP (AUVIS) to clear defects on a non Australian delivered car.
second, borrow air box.
third, as stated, doesn't have to be factory exhaust, so long as under 90db at 75% rpm of peak power (iirc).
and a working cat fitted.
shove a massive wad of steel wool to quieten down if needed, pass test, remove and play on....