Natural gas resources have been attracting attention as a major fuel in the future due to their feature of reducing environmental impact and a strongly-secured energy supply. The KU gas engine series of Mitsubishi Heavy Industries Group has pursued high efficiency and high output since the introduction of the KU30G gas engine to the market, with the total operating time already exceeding 9 million hours. The engines of this series ensure high reliability by sharing a common design for 80% of their components. This report presents technologies to make the most of the benefits of the product series to remodel a gas engine that has been in operation for a long period of time to the latest model during regular maintenance, which contributes to improving power generation efficiency and operability, while also introducing an example thereof.