The hydrogen (H2) internal combustion engine (ICE) is emerging as an attractive low life-cycle carbon powertrain configuration for applications that require high power, high duty cycle operation. Owing to the relative ease of conversion of heavy duty (HD) diesel ICEs to H2 and the potential for low exhaust emissions, H2 ICEs are expected to play a strong role in rapidly decarbonizing hard-to-electrify markets such as off-road, rail, and marine. The conversion of HD diesel ICEs to spark ignited H2 with port fuel injection is typically accompanied by a de-rating of engine power and torque. This is due to several fuel- and system-related challenges, including the high risk of abnormal combustion resulting from the low auto-ignition energy threshold of H2, and boost system requirements for highly dilute operation that is used to partially mitigate this abnormal combustion risk. However, HD ICEs must be adapted to a diverse range of vehicle applications, and so increasing ICE displacement to accommodate the de-rating challenge is not a feasible solution. This study details the research and development of a high power, ultra-low nitrogen oxides (NOx) emissions HD H2 ICE. The engine, converted from a diesel base, leverages an active pre-chamber ignition system to promote stable dilution limit extension, which lowers combustion temperatures and in-cylinder surface temperatures to reduce abnormal combustion. The ignition system also reduces instability-induced abnormal combustion risk. The resulting H2 ICE achieves power and torque levels consistent with those of the base diesel ICE, eliminating de-rating. The additional lean stability, especially during transient operation, leads to ultra-low cycle-average NOx emissions, achieving engine-out NOx of 0.24 g/kWh on a non-road transient cycle (NRTC) with a preliminary transient calibration. Test data was used to correlate a computational fluid dynamics (CFD) model of the engine, developing a simulation toolset that will be used to guide future optimization of the engine.