With respect to some of the previous posters, there are a lot of myths out there, that aren't even close to true. Any bike built before 1988 (i.e, all the ones you mentioned in your original post) incurs no GST.
Don't buy an incomplete bike, or a bike with the engine out, or a bike with no title, you're just wasting your time and money. Buy the best bike you can afford, with a title, and budget $1,000 - $1500.00 for shipping and various government charges on arrival.
I imported a CB750K0 that I bought from a forum member in 2007, and it wasn't hard. I bought the K0 for $3000.00, and Andy (736cc) dropped the bike out at the shippers near one of New York's airports for me. I got an import permit which cost me around 60 bucks.
I had the bike air freighted out here which cost $2000.00 US, which included cleaning, and the construction of a special steel pallet. (don't bother getting it crated, some wooden crates won't be allowed in some countries due to the possibility of insect infestation etc)
There's a really good website called Uship.com, its basically an auction that you create where shippers bid on your job. When I finish paying Larry (Momo here) for his CZ250 Enduro, I'll get it out here (pickup from Ringwood area) for just under $1200.00, including shipping, cleaning, import permits, etc.
Apart from the fact that bikes in the US are cheap, they're generally in much better condition due to most of America's short riding season. (the 1974 CZ I'm buying has less than 600 miles on it, amazing!) If you need any help mate, shoot me a PM. Cheers, Terry.