Transactive Energy Systems (TES) have created appealing opportunities to improve grid operation and decentralize management. TES advantages have been theorized for different market models at all grid scales. However, it is imperative to develop proofs-of-concept simulations and test the new management schemes before implementing them with actual users. In this regard, this paper presents a modular co-simulation engine for TES forward markets. The engine comprises an event-oriented software architecture for controlling simulations and an automatic modeling process for creating case studies. An orchestration tool helps reduce the overhead in distributed computer clusters. In addition, the proposed architecture involves a database manager for reporting purposes. This co-simulation engine was tested using a straightforward TES local market with a coalitional game configuration. In such a scenario, several residential users participate as followers communicating to a single demand aggregator. Simulation results show the impact of computer cluster configuration on market clearing times. The methodical simulation insights will help TES designers and regulators to develop robust mechanisms and analyze implementation scenarios.