In all honesty, I have seen cars get registered with restamped engines, even with just one number changed to hide a larger displacement engine.
Not saying to do anything illegal but unless the number used is already registered in another vehicle or flagged from a stolen vehicle its never a problem. You just need evidence to show you acquired the engine legally from a previous owner.
As mentioned earlier have a chat with your local compliance station see what they say, may just be the case of filling out a stat dec. If it ends up to hard or a no go and I had a collection of spare engines I could prove I bought that have legit engine numbers, well........... I do live in the NT and we are said to be a bit behind times so that may influence my decission, might be different elsewhere haha.
