Sue has informed me that they will not be returning my heads
The heads are your property. They are not entitled to keep them. Period. I'd give them a solicitors letter and a deadline.
If you wanted an expert witness, there are a few guys from the engine labs of Toyota, Holden, Ford & Orbital that would be well qualified. I think you'll struggle to find an experienced engine calibration guy outside of Melb (or Perth for Orbital). For the purpose of preparing for a potential court appearance, I'd stick with a professional engineer.