The process of crystallization is difficult to observe for transported, out-of-equilibrium sys-tems, as the continuous energy injection increases activity and competes with ordering. Inemergingfields such as microfluidics and active matter, the formation of long-range order isoften frustrated by the presence of hydrodynamics. Here we show that a population ofcolloidal rollers assembled by magneticfields into large-scale propelling carpets can formperfect crystalline materials upon suitable balance between magnetism and hydrodynamics.We demonstrate afield-tunable annealing protocol based on a controlled colloidalflow abovethe carpet that enables complete crystallization after a few seconds of propulsion. Thestructural transition from a disordered to a crystalline carpet phase is captured via spatial andtemporal correlation functions. Ourfindings unveil a novel pathway to magnetically annealclusters of propelling particles, bridging driven systems with crystallization and freezing inmaterial science.