Purcell effect enhancement of spontaneous emission rates is demonstrated for isoelectronic trap single-photon emitters. Two-dimensional photonic crystal slabs with L3 defects were fabricated in nitrogen delta-doped GaAs. Photoluminescence spectra of each photonic crystal cavity had a series of sharp and bright lines arising from individual nitrogen luminescence centers, which was confirmed by Hanbury-Brown and Twiss measurements. The emission rates of these lines depended on cavity detuning, indicating a resonant character of the enhancement. The observed emission lifetime in the cavity was 400 ps, which would be the shortest lifetime reported so far for luminescence centers in GaAs.