Solar flares are associated with intense X-ray emission generated by hot flaring plasma and by energetic particles in coronal magnetic loops. We investigate the temporal, spatial and spectral evolution of the properties of the X-ray emission produced in simulated kink-unstable magnetic flux-ropes (using MHD and test-particle methods). The numerical setup used consists of highly twisted coronal loops embedded in regions of untwisted background coronal magnetic field. The magnetic flux-rope reconnects with the background flux after the triggering of the kink instability and is then allowed to relax to a lower energy state. Strong ohmic heating leads to strong and quick heating (up to more than 15 MK), to a strong peak of soft X-ray (thermal) emission and to the hardening of the X-ray spectrum. Particles are accelerated in all the flaring loop volume, but the associated synthetic hard X-ray emission is nevertheless concentrated near the footpoints. The amount of twist deduced from the thermal X-ray emission alone is considerably lower than the maximum twist in the simulated flux-ropes. The flux-rope plasma becomes strongly multi-thermal during the flaring episode, and the emission measure evolves into a bi-modal distribution as a function of temperature during the saturation phase, and later converges to the power-law distribution during the relaxation/cooling phase.