Engine rebuilds normally come with 12 months or 20,000kms warranty. You will have to check the receipt for the Terms and Conditions on the engine rebuild. I don't beleive you have any legal recourse with the previous owner. "Buyer beware". Car yards are a different matter, obviously.
If it does fall within the warranty period the engine builder has first right of repairing the issue. You will need to speak with them and if no luck write a factual letter, being polite, but a letter of demand none the less. If that fails to motivate the engine builder/shop to come to the party organise a fair trading (S.A is something like Office of Buisness Consumer OBC or something similar) certified engineer to be on hand when the engine is removed. You will need to engage them to write a report, with pics and root cause of failure. Cost $600 to $1000. Based on the report - it's off to concilliation/arbitration hearing with the Shop.
An award will be made.
If the shop refuses to pay and is a memeber of MTA then MTA will pay you and deal with the shop.
The issue with shops paying is that they can ask to pay a small amount on a regular basis and generally the arbitrator will say yes.
No one really wins. Thats the whole point of the arbitration process.
Cheers
$19k is a lot to be stung.
I'm not sure how long you have owend nor how you have treated your GTR since owning it.
But dude, owners know if there car isn't right.
Engine builders - fark me - it's extremely hard to find a good one. Good ones will stand by their work and come to the party to identify the fault.