I would also second that as a bit of casual detective work, someone had, just had to know he built it. The first things any typical burglar has in mind are cash and jewelry, easy to take, know what they're looking for and can be in and out very quickly. Imagine the timescale on what they're trying to achieve, probably around 20 minutes, they can't afford to take unnecessary risks on items possibly too heavy to steal. Not only that, but they're unlikely to realize the market value for a engine. It would've been hard to pass on locally and quickly unlike other priced commodities.
You can choose to deny that if you want, and simply believe it was a completely random act, it's possible after all. That line of thinking doesn't really give you anything to go on though.
The likelihood is as awful as it may sound, someone he trusted, or he procured goods from to complete the engine betrayed him, knew he made a good job of it and stabbed him in the back. And that is traceable, because your father will know who his friends were at the time. It would've been the people in the immediate fallout of the event who will know the most about it. If it were me this would be my line of enquiry, a scatter gun isn't likely to yield results this far after. But thats just my opinion.
Michael W
Edited By Michael Walters on 05/07/2016 00:45:20