Can a small UAV restart its engine in mid-flight?
Some gliders have pop-up engines to extend range if they cant't catch rising air (known as turbo-sustainers) which start by in-flight windmilling, but I wouldn't count on it with an engine not designed to be started this way. Additionally, if the shutoff was not intentional it seems likely that whatever caused the engine to fail will make it even harder to start in this manner.