The Toyota 18RG engine is a 1.8-liter inline-four gasoline engine that was produced from 1979 to 1985. It was used in a variety of Toyota vehicles, including the Celica, Corona, and MR2.
While it is possible to fit a Toyota 18RG engine to a 1982 Lite Ace KM20, it would require a significant amount of modification. The engine mounts, transmission, and driveshaft would all need to be modified to fit the new engine. Additionally, the fuel system and electrical system would need to be upgraded to support the more powerful engine.
Due to the complexity and cost of the conversion, it is not a common modification. There are other, more easily installed engines that are popular for use in the Lite Ace KM20, such as the Toyota 2RZ-FE engine from the late-model Tacoma.