This paper is part of a broader research project aiming at studying, designing, and prototyping a hydrogen-powered internal combustion engine to achieve fast market implementation, reduced greenhouse gas emissions, and sustainable costs. The ability to provide a fast market implementation is linked to the fact that the technological solution would exploit the existing production chain of internal combustion engines. Regarding the technological point of view, the hydrogen engine will be a monofuel engine re-designed based on a diesel-powered engine. The redesign involves specific modifications to critical subsystems, including combustion systems, injection, ignition, exhaust gas recirculation, and exhaust gas aftertreatment. Notably, adaptations include the customization of the cylinder head for controlled ignition, optimization of camshaft profiles, and evaluation of the intake system. The implementation incorporates additive manufacturing for the production of new intake manifolds and a new turbocharger in order to optimize the volumetric efficiency of the new hydrogen engine. The project is targeting a wide range of applications (automotive, cogeneration, maritime, off-road, railroad, etc.). This paper focuses on the Life Cycle Assessment (LCA) of the diesel-powered engine and preliminary evaluates the effects of its conversion into a hydrogen-powered engine in terms of environmental impacts. The LCA system boundary is cradle-to-grave, and the assessment is entirely based on primary data (i.e., company-specific material and energy flows are used), which is one of the main novelties of this article. The results show that climate change, use of fossil resources, freshwater ecotoxicity, acidification, and particulate matter are the five most relevant impact categories. The diesel engine results in a carbon footprint of 0.36 kg CO2eq/km, with the use phase being the main contributor to the whole life cycle, as expected. In terms of climate change, the preliminary LCA evaluation of the hydrogen engine demonstrates that hydrogen may be a valid solution if produced from certain production routes, i.e., considering steam methane reforming and coal gasification combined with carbon capture storage systems.