Why didn't you take the plate off his car?
Is the car still registered? If it is don't worry, just drive it.
If you can prove that a car has been registered in Australia you do not need a compliance plate. You just need the engine number, the chassis number and proof of previous rego.
New Falcons do not have a complaince plate, they have a sticker. It is designed to fall apart if you try to peel it off. (to stop people re-birthing them) The problem is that the stickers fall off when you de-grease the engine!
You could just find another plate and radiator panel

but then you need the receipt for the car. (the one the plate came from)