Two different issues there... Engineer’s certificate is required for registration not insurance.
Insurance companies will take your money no matter what. It's only when need to claim from them that they start to look at modifications, licence points etc. Anything so they don't have to pay out.
Insurance is a false sense of security these days.
What was very evident in the floods in Brisbane this year is not all insurance companies are equal! Only one (1) insurance company stood by its flood cover. Every other one hid behind some "rising water" clause and hired hydrological engineers to prove it wasn't a flood, it was rising water.
Back to car insurance... I figured that I may pay a little bit more for my insurance with Shannons but the choice of repairer and a target customer base of enthusiasts with modified cars (I think) means they may be more inclined to honour a claim provided I have 100% disclosure to them! Hence why I sent them everything I had on the modifications to my car and I agreed a reasonable value (less than its replacement value and worth IMO but acceptable to me) in the event of a total loss.
Unless you are 100% stock, forget about insurance with the regular insurers. Secondly, read a few Australian forums on experiences with insurers of prestige or rare cars in the event of a claim. Pick one with your eyes wide open. Then I'd take the car to them and have them inspect it and photograph it as you would a piece of jewellery for their records.
Insurance in Australia is almost as bad paying into a Nigerian scam. You can bet that they are going to try to do everything they legally can to weasel out of the contract in the event of a total loss claim!