In most states in the US registration is not a problem. There are only a few that require an emission inspection and almost none that require a safety inspection any longer. There are many Big Block Vegas and Pintos (not sure if you had those chassis styles in Australia or what they might have been called) and having a 500 to 600 HP supercharged or turbocharged whatever is only regulated by how much you are willing/able to spend on your car. It becomes a little bit tricky in those few states where emission laws rule our lives but even in most of those states that is a regional thing (certain counties, etc) and imaginitive folks seem to find a way to license their cars. In the US if you wanted a Viper V-10 in your 1200 the only thing that would stop you is how to make it fit. (Might I suggest a Toronado transaxle ane rear-mid engine positioning?)