From the article linked above
The 52-year-old Toowoomba and Lockyer Valley identity appeared to be attempting to return to the Emu Gully airstrip when a mechanical failure sent the plane plummeting to earth.
Is it more likely the problem was the reason for the return and control of the aircraft was lost executing the return, perhaps attempting the "ïmpossible turn"?
Automotive engines haven't the best reliability in aircraft use and neither have gearboxes/reduction drives on engines with less than 9 cylinders been all that successful in aircraft. It sounds like this aircraft had both.
Two ag outfits over here each spent a kings ransom developing their own V8 engines (one a big block GM the other Ford) plus gearbox setups to replace the venerable IO720, In the end they gave up.