  While it has been suggested that there is a connection between the magnetic
properties and the internal structure of young stars, there have not been
enough magnetic measurements to firmly establish such a correlation at the
earliest ages. Here, we contribute to this endeavor by presenting stellar
parameters and magnetic field strength measurements of BP Tau and V347 Aur,
both stars observed with the near-infrared spectrograph iSHELL. We first test
the accuracy of our method by fitting synthetic stellar spectra to a sample of
nine main and post-main-sequence stars. We report uncertainties of $\sigma_{\rm
Teff}$ = 91 K in temperature and $\sigma_{\rm log(g)}$ = 0.14 in gravity. We
then apply the modeling technique to BP Tau and measure a surface magnetic
field strength of $\langle \rm B \rangle $ = 2.5$^{+0.15}_{-0.16}$ kG,
confirming literature results. For this star, however, we obtain a much lower
temperature value than previous optical studies ($\Delta \rm T \sim 400$ K) and
interpret this significant temperature difference as due to the relatively
higher impact of starspots at near-infrared wavelengths than at optical
wavelengths. We further apply this technique to the class I protostellar source
V347 Aur and measure for the first time its magnetic field strength $\langle
\rm B \rangle = $ 1.36$^{+0.06}_{-0.05}$ kG and its surface gravity log(g) =
3.25$^{+0.14}_{-0.14}$. Lastly, we combine our measurements with
pre-main-sequence stellar evolutionary models and illustrate the effects
produced by starspots on the retrieved masses and ages of young stars.
