ScummVM contains almost 80 game engines (and a lot of sub-engines) supporting hundreds of games. Most of them are made by straight reverse engineering of the original executables.

In the early days we did have a lengthy back-and-forth with a tenacious lawyer from LucasLegal but they backed off when we convinced them that we didn't support piracy.

It is not uncommon for rereleases of older games to use the ScummVM engine instead of the original engine, see for instance the recent rerelease of Westwood's Blade Runner on GOG.com, which wouldn't have happened without ScummVM.