  Stellar engines are hypothesized megastructures that extract energy from the
host star, typically with the purpose of generating thrust and accelerating the
stellar system. We explore the maximum potential speeds that could be
realizable by stellar engines, and determine that speeds up to $\sim 0.1\,c$
might perhaps be attainable under optimal conditions. In contrast, natural
astrophysical phenomena in the Milky Way are very unlikely to produce such
speeds. Hence, astrometric surveys of hypervelocity stars may be utilized to
conduct commensal searches for high-speed stellar engines in the Milky Way. It
may be possible to derive bounds on their abundance, but this requires certain
assumptions regarding the spatiotemporal distribution of such engines, which
are not guaranteed to be valid.
