  The evolution of a pulsar wind nebula (PWN) inside a supernova remnant (SNR)
is sensitive to properties of the central neutron star, pulsar wind, progenitor
supernova, and interstellar medium. These properties are both difficult to
measure directly and critical for understanding the formation of neutron stars
and their interaction with the surrounding medium. In this paper, we determine
these properties for PWN G54.1+0.3 by fitting its observed properties with a
model for the dynamical and radiative evolution of a PWN inside an SNR. Our
modeling suggests that the progenitor of G54.1+0.3 was an isolated ~15-20 Solar
Mass star which exploded inside a massive star cluster, creating a neutron star
initially spinning with period ~30-80ms. We also find that >99.9% of the
pulsar's rotational energy is injected into the PWN as relativistic electrons
and positrons whose energy spectrum is well characterized by a broken
power-law. Lastly, we propose future observations which can both test the
validity of this model and better determine the properties of this source -- in
particular, its distance and the initial spin period of the central pulsar.
