  Magnetic fields can drastically change predictions of evolutionary models of
massive stars via mass-loss quenching, magnetic braking, and efficient angular
momentum transport, which we aim to quantify in this work. We use the MESA
software instrument to compute an extensive main-sequence grid of stellar
structure and evolution models, as well as isochrones, accounting for the
effects attributed to a surface fossil magnetic field. The grid is densely
populated in initial mass (3-60 M$_\odot$), surface equatorial magnetic field
strength (0-50 kG), and metallicity (representative of the Solar neighbourhood
and the Magellanic Clouds). We use two magnetic braking and two chemical mixing
schemes and compare the model predictions for slowly-rotating,
nitrogen-enriched ("Group 2") stars with observations in the Large Magellanic
Cloud. We quantify a range of initial field strengths that allow for producing
Group 2 stars and find that typical values (up to a few kG) lead to solutions.
Between the subgrids, we find notable departures in surface abundances and
evolutionary paths. In our magnetic models, chemical mixing is always less
efficient compared to non-magnetic models due to the rapid spin-down. We
identify that quasi-chemically homogeneous main sequence evolution by efficient
mixing could be prevented by fossil magnetic fields. We recommend comparing
this grid of evolutionary models with spectropolarimetric and spectroscopic
observations with the goals of i) revisiting the derived stellar parameters of
known magnetic stars, and ii) observationally constraining the uncertain
magnetic braking and chemical mixing schemes.
