I am not a purist at heart! Just wanted to get that out of the way first!!
But, when it comes to Classic cars and the restoration of them, the value is more if it is operating with the original parts and systems the car came with. Modified will always be modified. That doesn't mean you should not change your car. It is your car after all, and you are free to do whatever you wish with it. Just keep in the back of your mind that this will not help the cars market value in the long term. It's the same as with originality, compared to a fully restored car or any antique for that matter.
About cars being allowed into Australia. I believe this is because the Federal Government have installed a set of rules in the last few years, that require a car to be checked for Asbestos, even the slightest trace of it in a car. And the onus is on the owner to prove it has none. From what I have heard, this can be very expensive, if not prohibitive. I have a RHD car, bought in Australia in 1985 and then taken to Europe and Germany before finally being brought to the USA in 1992. Guess what? I don't believe that I can bring my car back into Australia, unless I tear a lot of it out, Clutch, Brakes, under floor Insulation and God only knows what else. If I did, my car would not be original anymore besides not justifying the cost of the modifications. Enough said....