In the mechanical and plant engineering industry, characterized by small and medium-sized enterprises (SMEs), timely commissioning of customized products is a decisive competitive factor. Successful implementation requires precise project planning and control, which depend on constantly updated information concerning all processes throughout the project execution phase. Alongside digitalization and its associated challenges of establishing a digital shadow for production and logistics processes in line with specific requirements, sustainability is becoming increasingly important. As suppliers, SMEs are indirectly affected by the statutory reporting obligations for CO2 balancing. In the future, they will also need to determine the Product Carbon Footprint (PCF) of their customized products to meet sustainability reporting requirements and secure competitive advantages. This article discusses the specific challenges involved in developing a digital shadow for one-of-a-kind and small-batch manufacturers in mechanical and plant engineering as well as its utilization to assess the CO2 emissions of customized products and outlines a methodological research approach.