In the global scenario marked by the increasing environmental awareness and the necessity on reducing pollutant emission to achieve the decarbonization goals, action plans are being proposed by policy makers to reduce the impact of the climate change, mainly affecting the sectors that most contribute to CO2 emissions such as transportation and power generation. In this sense, by virtue of the National Energy Plan 2050, the Brazilian market will undergo the decommissioning of thermal power plants fueled by diesel and heavy fuel oil (HFO) by 2030, compromising about 6.7 GW of power capacity according to the Brazilian Electricity Regulatory Agency (ANEEL) database. An alternative to the scrapping of these engine power plants is their conversion to operate with fuels with a lower carbon footprint, such as the natural gas. This work, therefore, aims to numerically assess the conversion feasibility of a HFO large bore four-stroke turbocharged engine to operate with natural gas by means of a one-dimensional engine modeling. First, the 1D non-converted engine model operating with HFO is validated with experimental data. Then, the conversion of the HFO engine to natural gas is carried out by adding a wastegate for the air-fuel ratio control, changing the compression ratio and the fuel injection, and introducing the pre-chamber ignition system. At this stage, the performance of the engine operating with most of its stock components is evaluated, including the presence of knock, fuel slip, and components that may not be suitable for NG operation and must be adapted, redesigned, or replaced. After that, modifications on the valve timings are proposed to reduce the methane slip and allows a proper scavenging. In conclusion, this study numerically assessed converting an HFO engine to natural gas, identifying new component specifications and presenting alternatives to maintain engine performance post-conversion.