* spotters are sometimes organised into clubs, with membership cards and all. Such a membership card should make a good impression, I think.
* I can't imagine a genuine spotter would object to being identifyable, in one way or another. OTOH there are often hobby photographers around airfields, who are not specifically into aircraft but just into fine sights.
* Any details of recent thefts? I heard similar reports from Germany, perhaps one or two years ago, and, even earlier, from France.
* Isn't there a web page (at rotax.www ?) where the serial numbers of stolen engines are listed?