My advice for the definition:
Keep it simple, and from preference, use another piece of already accepted legislation. A neat example is this one:
The Department of Infrastructure and Regional Development (DIRD, previously DoTaRs)
https://infrastructure.gov.au/vehicl...les/0-2-13.pdf
Definition:
"6.3 Replica: A vehicle that has a similar Make and/or Model name and/or the
appearance of another Model in order to gain a marketing position based on that
similarity. Typical examples are “AC Cobra”, “Lotus Seven” and “Jaguar XJ13”."
This keeps things simple for the definition as it allows you to build pretty much any vehicle as a replica.
Then use your state's Historical Registration Scheme to determine the age of something that you can build a replica of, for example, in NSW it's 30 years:
http://www.rms.nsw.gov.au/documents/...eclaration.pdf
So if a vehicle is >30 years old, and passes a silhouette test (i.e. similar in appearance) then it's a replica.
South Australia has a similar scheme but doesn't have a 'rolling 30 years' it has a fixed date:
https://www.sa.gov.au/topics/driving...l-registration
The fixed date is 01/01/1979.
Don't get bogged down in 'period authentic' rubbish otherwise we'll have to build exact copies - you need the freedom to change the chassis to make them stiffer for decent suspension, you need to be able to use readily available parts (i.e. engine, gearbox, hubs, brakes).
So, my advice is summarised:
1. Use pre-existing legislation
2. Keep it simple
If anything, I would suggest the 'rolling 30 years' vs the 'fixed date' approach, but as this is in your state, I would recommend using your own state's legislated date as a good start point, then push to get that changed to a rolling 30 years in place of the fixed date.
Treeve