We report on the challenges and limitations of direct coupling of the magnetic field from a circuit resonator to an electron spin bound to a donor potential. We propose a device consisting of a trilayer lumped-element superconducting resonator and a single donor implanted in enriched Si-28. The resonator impedance is significantly smaller than the practically achievable limit obtained with prevalent coplanar resonators. Furthermore, the resonator includes a nanoscale spiral inductor to spatially focus the magnetic field from the photons at the location of the implanted donor. The design promises an increase of approximately 2 orders of magnitude in the local magnetic field, and thus the spin-to-photon coupling rate g, compared with the estimated rate of coupling to the magnetic field of coplanar transmission line resonators. We show that by use of niobium (aluminum) as the resonator s superconductor and a single phosphorous (bismuth) atom as the donor, a coupling rate of g/2 pi = 0.24 MHz (0.39 MHz) can be achieved in the single-photon regime. For this hybrid cavity-quantum-electrodynamic system, such enhancement in g is sufficient to enter the strong-coupling regime.