Game engine recreation
Game engine recreations are remade engine interpreters for video games that replace the original engine binary that came with the original game. A notable example of game engine recreation is ScummVM which successfully recreated the SCUMM engine of classical LucasArts' point and click adventures, for further examples see list of game engine recreations.
Motivation
Game engine recreations are made to allow the usage of classical games with newer operating system versions, recent hardware or even completely different operating systems than originally intended. Another motivation is the ability to fix engine bugs which is often hard or impossible with the original engines (with notable exceptions, see community patch) once a software has become unsupported Abandonware, with the source code not available.
Methods
In many cases, these engine are created based on reverse engineering the original binary executable. A good example is OpenTTD. Occasionally, as was the case with some of the engines in ScummVM, the original developers have helped the projects by supplying the original source code—those can be then called source ports.
Alternatives
Emulation of classical systems or operating systems is an alternative to an engine recreation; for instance DOSbox is a notable emulator of the PC/MS-DOS environment.