No prob. I'm reasonably confident that there are other multiple projects that have also done this; I just tried to list what looked like the most-currently-viable ones.

kagis

The first I think I remember seeing chronologically was FIFE, which IIRC was renamed from some slightly-different acronym from when it was intended to only run Fallout games. It looks like they've focused on becoming a generic RPG engine:

https://www.fifengine.net/

FIFE is a free, open-source cross-platform game engine. It features hardware-accelerated 2D graphics, integrated GUI, audio support, lighting, map editor supporting top-down and isometric maps, pathfinding, virtual filesystem and more!

The core is written in C++ which means that it is highly portable. FIFE currently supports Windows, Linux and Mac.

Games utilizing FIFE are programmed through Python scripting layer on top of the base C++ API. Games can be also programmed using the C++ layer directly.

FIFE is open-sourced under the terms of the LGPL license so you can freely use it in non-commercial and commercial projects.

It sounds like they may have not taken it to full playability of the first two games; IIRC, the original intention was to do so:

https://falloutmod.fandom.com/wiki/FIFE

FIFE stands for Flexible Isometric Fallout-like Engine and is an open source project for the creation of cross platform ISO/top-down 2D games (e.g. RPGs & RTS'). The assets of Interplay's RPG classics Fallout 1 & 2 are supported as test implementation but are not required to work with FIFE. It is not a Fallout emulator and you cannot play Fallout with it. The project's goal is more universial. You can read graphics from fallout data files and create your own mods or draw you own content and make a completely new game.

Then there's Falltergeist:

https://github.com/falltergeist/falltergeist

Falltergeist is an opensource alternative for Fallout 2 and Fallout 1 game engines. It uses C++, SDL and OpenGL. Falltergeist requires original Fallout resources to work.

But the last GitHub commit was three years ago, and the main site's last blog update was in 2018.

There's darkfo:

https://github.com/darkf/darkfo

A post-nuclear RPG remake

This is a modern reimplementation of the engine of the video game Fallout 2, as well as a personal research project into the feasibility of doing such.

It is written primarily in TypeScript and Python, and targets a modern (HTML 5) Web browser.

However, the last commit was six years ago.

There's Harold, which is apparently a project continuing darkfo:

https://github.com/OldGamesLab/Harold

The project is based on darkfo codebase, but is modernized for Python 3, potentially with more improvements and bug fixes coming in the future.

Its last commit was three years ago.

There's Fallout Equestria Reloaded

which apparently is some sort of unholy mating between My Little Pony and Fallout:

https://github.com/Plaristote/fallout-equestria-reloaded

Qt-based game engine for Fallout-like RPGs, developed for the Fallout Equestria RPG project

I don't think that the goal was so much to play Fallout as to use the assets to bootstrap a playable MLP RPG.

There have been commits in the past two months, so apparently someone is actually seriously plugging away.

Then there's FOnline, another engine reimplementation, this one intended to be played multiplayer online:

https://github.com/cvet/fonline

Looks active.

https://www.fonline-reloaded.net/

FOnline: Reloaded is a free to play post-nuclear MMORPG based on FOnline: 2238, an award-winning game set three years before the events of Fallout 2. FOnline: Reloaded provides you with a unique opportunity to revisit the ruins of California and explore the familiar locales from Fallout 1 and Fallout 2.

FOnline: Reloaded is a player-driven, persistent world MMORPG that allows you to participate in a wide range of activities, which range from faction wars to exploration, mining, scavenging for resources, caravan raids and more. The game puts a lot of emphasis on team play and dynamic, unscripted PvP action, but there is absolutely nothing to stop you from focusing on PvE dungeons or role-play.

FOnline: Reloaded is powered by the latest iteration of the FOnline Engine, which was created from scratch by Cvet and which is capable of utilizing assets imported from the original Fallout games, as well as Fallout: Tactics, Arcanum and Baldur’s Gate. The development of this engine started back in 2004 and continues to this day.