  We present results from a semicoherent search for continuous gravitational
waves from the low-mass X-ray binary Scorpius X-1, using a hidden Markov model
(HMM) to track spin wandering. This search improves on previous HMM-based
searches of LIGO data by using an improved frequency domain matched filter, the
$\mathcal{J}$-statistic, and by analysing data from Advanced LIGO's second
observing run. In the frequency range searched, from $60$ to
$650\,\mathrm{Hz}$, we find no evidence of gravitational radiation. At
$194.6\,\mathrm{Hz}$, the most sensitive search frequency, we report an upper
limit on gravitational wave strain (at 95\% confidence) of $h_0^{95\%} = 3.47
\times 10^{-25}$ when marginalising over source inclination angle. This is the
most sensitive search for Scorpius X-1, to date, that is specifically designed
to be robust in the presence of spin wandering.
