  We report an updated mass and magnification model of galaxy cluster Abell 370
using new NIRCam and NIRISS data from the CAnadian NIRISS Unbiased Cluster
Survey (CANUCS). Using Lenstool and a combination of archival HST and MUSE data
with new JWST data as constraints, we derive an improved gravitational lensing
model and extract magnifications of background galaxies with uncertainties.
Using our best fit model, we perform a search for new multiply imaged systems
via predicted positions. We report no new multiply imaged systems with
identifiable redshifts, likely due to already very deep HST and Spitzer data,
but confirm a $z\sim8$ multiply imaged system by measuring its redshift with
NIRISS and NIRSpec spectra. We find that the overall shape of the critical
curve for a source at $z = 9.0$ is similar to previous models of Abell 370,
with small changes. We investigate the $z\sim8$ galaxy with two images
observable with an apparent magnitude in the F125W band of $26.0\pm0.2$ and
$25.6\pm0.1$. After correcting for the magnifications of the images,
7.2$^{+0.2}_{-1.2}$ and 8.7$^{+0.4}_{-0.4}$, we use SED fitting to find an
intrinsic stellar mass of log($M^*/M_{\odot})$ = 7.35$^{+0.04}_{-0.05}$,
intrinsic SFR of 3.5$^{+2.2}_{-1.4}$ M$_{\odot}$/yr, and $M_{UV}$ of
-21.3$^{+0.2}_{-0.2}$, which is close to the knee of the luminosity function at
that redshift. Our model, and corresponding magnification, shear, and
convergence maps are available on request and will be made publicly available
on MAST in a CANUCS data release (DOI: 10.17909/ph4n-6n76).
