Achieving Net Zero aviation objectives demands a fundamental rethinking of aircraft propulsion architectures. The design of electric powertrains, where strong couplings between propulsion, electrical and thermal management systems are inadequately captured by conventional sequential sizing approaches represents the key technological challenge. In such architectures, Balance of Plant (BoP) components become critical design drivers, as cooling requirements directly impact aerodynamic drag and auxiliary power demand, with a straight penalty on propulsive efficiency. The objective of this work is to develop an integrated, physics-based sizing and Multidisciplinary Design Optimisation (MDO) framework for H₂-fuelled electric power trains in general aviation propeller aircraft, capable of considering these couplings at the conceptual design phase. A novel holistic optimisation environment is developed where fuel cell, electric motor, propeller, air intake, and thermal management system are simultaneously sized within a unified framework leveraging consistent physical models. This tool is implemented in OpenMDAO, enabling tight coupling between disciplinary models and efficient gradient-based optimisation. The fuel cell system is modelled using Proton Exchange Membrane Fuel Cell (PEMFC) polarisation curve and electrochemical constitutive equations, computing stack voltage, thermal balance, reactant mass flows, and also sizing ancillary elements. BoP power consumption is explicitly calculated and included within the overall power balance. Thermal management system includes the sizing of heat exchangers and their cooling air path. The air intake is modelled and optimised using a one-dimensional formulation based on compressible flow relations, combining Rayleigh, Fanno and isentropic flow effects while estimating pressure losses induced by the heat exchanger and external drag. The methodology follows an incremental development strategy, with individual components validated prior to integration into the MDO framework. A Single-Design-Point optimisation is used for baseline sizing and extended to a Multi-Design-Point formulation to ensure constraint fulfilment across the flight envelope, avoiding infeasible or inefficient designs. The resulting framework will provide a consistent foundation for the preliminary sizing of H₂-electric powerplants, supporting the development of viable hybrid-electric aircraft aligned with long-term sustainability objectives.