Hydrogen-powered mobility is believed to be crucial in the future, as hydrogen constitutes a promising solution to make up for the non-programmable character of the renewable energy sources. In this context, the hydrogen-fueled internal combustion engine represents one of the suitable technical solution for the future sustainable mobility. In a short-term perspective, the development of the green hydrogen production capability and distribution infrastructure do not allow a substantial penetration of pure hydrogen IC engines. For this reason, natural gas – hydrogen blends can represent a first significant step towards decarbonization, also determining a trigger effect on the hydrogen market development. The present paper is focused on the analysis of the combustion and performance characteristics of a production PFI natural gas engine, run on blends with 15% in volume of hydrogen (HCNG). More specifically, a fuel-flexible, predictive 1D simulation model has been developed within the Gasdyn code and validated against experimental data, under natural-gas operation. The model features the application of a predictive combustion model, which intrinsically accounts for the fuel burning characteristics through the specification of its laminar burning speed. The model has then been applied to the HCNG fueling case, with no changes in the combustion model settings, and showed rather good accuracy, thus demonstrating its fuel flexibility. The model was then used as a digital twin of the real engine, and the engine calibration was optimized in order to fully exploit the potential benefits of the hydrogen blending, with no or minor modification of the engine design with respect to a production spark ignition engine.