Don't bother without another used GZE engine, the price they attract is barely justifiable considering it will probably need a rebuild in due time also.
If your current GZE is reusable, take it to an engine builder, if you had 3k budget for someone to rebuild it for you, I'd expect you should just have enough. Save yourself a lot of money by dropping the engine yourself.
If you want to save even more, you can do the engine teardown yourself. It's not overly hard with the right tools; a quality set of sockets, both 6 and 12 point, deep socket is better for the 12 point too, a breaker bar for those sockets, and a torque wrench.
Download the BGB and have a good read of it. If you can do the engine drop, teardown and rebuild yourself, then you only need $1500 for a machine shop to recondition the engine for you and grab a few parts to give it a major service (water & oil pump, timing belt and tensioner).
With a fresh engine, I would then HIGHLY recommend you consider an aftermarket ECU and reputable tuner. It's likely your engine started knocking due to lean out associated with the mods performed on it. An aftermarket ECU will ensure your engine is running its best, and you will find you'll net yourself a very good power upgrade.
I can't advise any good engine builders in sydney but I know there are people on here who can help. For aftermarket ECU I recommend chatting to Adaptronic, they are local, very helpful and can advise of good tuners, they are fairly cheap too! Quite a few of the AW11 guys run them and get good results, whether its turbo, SC12 or SC14 with W2A.