From WikiPedia,
A
Department of Transport (DOT) investigation into Valentich's disappearance was unable to determine the cause, but that it was "presumed fatal" for Valentich.
[2] Five years after Valentich's aircraft went missing, an
engine cowl flap was found washed ashore on
Flinders Island. In July 1983, the
Bureau of Air Safety Investigation asked the Royal Australian Navy Research Laboratory (RANRL) about the likelihood that the cowl flap might have "travelled" to its ultimate position from the region where the aircraft disappeared. The bureau noted that "the part has been identified as having come from a Cessna 182 aircraft between a certain range of serial numbers" which included Valentich's aircraft.
What would be prudent would be to find out the serial number of the 182 that was damaged on Flinders was within the range of the cowl flap that was found. It also sounds like a bit of a stretch that based on his departure time etc that the cowl flap could have made it all the way to Flinders island over a period of 5 years (I think it was).