Q: Wrong starter motor fitted by a garage

If you have the starter motor (the wrong one), the easiest way to prove your case is to get from the manufacturer, or re-manufacturer, the cataloging information. In other words, the "part" should have a part number and that part number is going to be uniquely associated with applications (vehicles, year, make and model). Presumably your year, make and model, including engine, are NOT listed by the starter manufacturer as an application for the starter that the Mechanic mistakenly installed. The next step is to use that same catalog to identify what is indeed the correct part number for your vehicle and just simply show the court that that correct number is indeed not the part that was installed on your vehicle. A mechanic can certainly serve as an expert witness to confirm your data (depending on your local rules of court). Incidentally, you are reminding me of my first such like case which occurred in the year 1977. A car dealer sold me the wrong sliding yoke for a manual transmission on a 1966 Ford Falcon. I bought the part and had stuck it on a shelf in the garage for later installation, only to discover months later when I went to install it that the dealer had sold me the completely wrong part. I was 17 years old at the time but even at that point I had observed that that sort of outcome was a habitual problem in the world of car parts. That made me irate, the fact that this was just a continuing, unresolved problem, and so I sued the car dealership and won. I then had to jump through a lot of hoops to enforce the judgement against the car dealer. I do remember eventually getting my money, though, after a Court Bailiff got involved (they enforced judgments at that time) and you will too if you file the case and properly prosecute it. If we can provide other assistance, let us know.