I am not sure I would buy a new aircraft replacement at this stage given that 12 of the F models have already been pre-wired as growlers. It is quite possible that the F model will be replaced sooner rather than later so I would look at the possibility of converting at least one of them to a Growler.
I think the answer to the question, will Govt / Defence / RAAF seek a replacement for the lost Growler, is still very much "how long is a piece of string".
We don't know if the original procurement of 12 Growlers included attrition or not (if I remember correctly, in the early days the Govt was looking at six Growler kits). I think a lot will depend on when the Growlers reach FOC and what we learn of the capability between now and then, and of course what might, or might not, happens to the Super Hornet fleet (will they stay in service or be replaced with the 4th Squadron of F-35As, for example) and of course what 'potential' compensation can be sought and received from the manufacturers regarding the failure with the engine too.
If and I say 'if', there is a decision to replace the lost Growler sooner rather than later, I can see three options:
1. Order a brand new airframe from Boeing.
2. Convert one of the 'pre wired' Super Hornets, or
3. And this is my personal preferred option, see if a deal can be struck with the USN for a replacement Growler from exactly the same production lot as the RAAFs Growlers.
Why do I prefer my option 3?
It should be a relatively quick process and the airframe is of the exact same age and production time as that of the RAAF fleet, a new airframe will be 'out of step' with the rest of the fleet, maybe production changes between Lots that could cause support or maintenance issues?
And as for converting a 'one off' of the pre wired airframes, it will have a lot more hours on the airframe (far earlier production Lot), and we have no idea how 'expensive' a one off conversion/update could be.
If at some point in the future the Govt decides to replace the Super Hornets with the last batch of F-35A, and also decides to expand the Growler fleet, then there are probably 'economies of scale' in converting a larger number of Super Hornets to Growlers (6-12 airframes?).
Anyway, just my opinion of course too.
Cheers,