I'm in WA, fyi.
I bought a 65 Pan in from the states, and because it had a 61 engine in it and the USA title their bikes on engine number, I had to register it in WA as 1961 Pan.
Even though I had a brand new S&S P93 engine in it, with it's own unique engine numbers, before I could leave the rego depot, I had to stamp into frame the 1961 engine number, was my choice where to put it, just had to be done for WA rego purposes.
I plan to build a chopper at some stage, my plan is to use the original 61 PAN cases I have, build the bike, never rego it. Use my current plate of my 65/61, lol, and if I get stickered I'll just take the 65/61 in to get it removed.
In this case it helps that I have more than 1 bike and get the same number onto two bikes legit, sort of. does this make sense, hope not. LOL.
If you're going to build a Chopper in WA from the ground up using a custom frame you're going to have to read up and learn all about ICVs (Individually Constructed Vehicle) as that is what you're Chopper will be in the eyes of the transport department of WA.
As an ICV you'll need to fully comply with the ADRs (Australian Design Rules) of the day, being 2017 right now, regardless of the year of manufacture of the engine. The ADRs spell out very specifically what you can and cannot do, and the measurements of everything you must comply with, in terms of frame, handlebar width and height, how far the front and rear fenders have to cover the wheels, all the lighting, indicators, reflectors, license plate position, exhaust volume, mirror surface area, ABSOLUTELY EVERYTHING! And as mentioned above the big killer to your Sweedish Chopper dreams (and my same dreams) is the 550mm rule. This is a hard and fast rule in WA for all motorcycle regardless of ADRs or year of manufacture, with NO exceptions PERIOD.
You'll also need to get an accredited engineer involved in the project as they will have to sign off on every aspect of the bike before you can take it to the transport department for licensing inspection. They have to brake test the bike, noise test the bike, sign off on welds all over the bike, or if the frame doesn't have an MSO they'll have to approve the frame (good luck with that).
The easier paths (both of which I have taken), is (A) to buy a bike already licensed and then change it to suit your style, or (B) build a bike using a recognised vintage frame (I'm starting a Chopper build with a 1965 Triumph frame) that you know will be licensed as a vintage bike so you avoid all the ADRs rubbish like indicators etc, but in all cases you cannot escape the 550mm rule in WA unfortunately.