Reusing structural elements is an effective approach to designing low-carbon structures. However, integrating it into conventional design processes without compromising design freedom is challenging, as designing with a given inventory of elements significantly impacts the resulting design. This paper presents a new computational design framework for creating reticular structures composed of both reused and new elements. The framework combines interactive equilibrium design with stock-constrained assignment optimisation, allowing the exploration of diverse typologies that are structurally sound and informed by the characteristics of the stock. To this end, custom layout and geometry optimisations based on constraint projections are combined with a framework for optimally assigning reclaimed and new structural elements. The entire framework is conceived for interactive design through real-time computations. The applicability of the method is showcased by a realistic case study using a real-world stock of reclaimed steel elements.