Personally, I'd put the thermometers on the bottom engine as, like multi-row radials, getting cooling air to the back of the engine (bottom in this case) can be difficult, especially as all the cylinder heads are aligned. It looks like the engines are connected to the rotor shaft via chains; if one engine fails, does the shaft lock up or are there free wheeling devices? There was a guy, Manolis IIRC, who used to post on F1 Technical about his 2 stroke flying pogo stick; this actually looks safer.