Its rather simple to answer. Look HERE and find the engineer that is closest to you, or any that you can visit and ask them what is the biggest engine they will engineer that will meet the DOT's approval. An engineer can engineer any size motor in any car, but this does not meet the DOT's rules. Make sure to state that it is for a legally road registered vehicle.

1.4 litre is the biggest size engine most Australian state DOT's will allow you to fit without engineers report.
Dont bother with what the DOT tell you. I once visited a NSW RTA branch to pay for rego on a datsun that had earlier that day been Certified by an Engineering Signatory, and the person at the counter in the RTA branch didnt know how to process the registration because they didnt know what an engineers certificate was.