Examining the effects of converting marine diesel engines to methanol engines in terms of economics, the environment, and forthcoming regulatory requirements is the aim of this study. The article stands out by using an assessment that looks at how the retrofit application affects meeting Energy Efficiency Existing Ship Index (EEXI) and Carbon Intensity Indicator (CII) criteria as well as an economic and environmental analysis of retrofitting a ship with a methanol engine based on present circumstances and projections for the future. An oceangoing container ship's noon reports for a forty-day cruise have been obtained to perform the analysis. Two different methanol engine retrofit scenarios having smaller (ME1) and larger (ME2) power outputs have been evaluated. Results demonstrated that both ME1 and ME2 engines have considerably reduced operational carbon dioxide (CO2) emissions and their CII values are 9.893 and 9.674 respectively. The ME1 with a 12.259 value has great performance in terms of EEXI which is quite significant since the EEXI of the reference container is 21.272. The usage of renewable (RN) methanol is found highly critical in terms of lifetime (LT) emissions. Despite falling behind in the electricity production cost (EPC) metric, ME1 generally outperformed ME2 in economic comparison, and the payback period (PBP) is calculated at 9.63 years with 2023 prices. The economic model forecast that PBP can be reduced to 4.44 years for the same scenario with the possible developments in methanol production and distribution technologies by 2050.