[paavohtl]

The source is only partially available; they released the source code of the game logic, but the engine was not included in the source release. You'd need to reverse engineer & remake the engine to make any major improvements to the game, such as porting it to new platforms.

[keyringlight]

This sounds like how most moddable games of the era released their SDKs. You could make a mod that changed gameplay and/or assets, but you needed the needed the original game itself for the engine and original assets as a foundation/runtime. One step further would be a 'total conversion' that replaced all the assets but you still needed the engine, and then you get later releases like the id GPL releases of the engine so you could have engine+gameplay+content all by yourself. Even then you'd still need to abide by the license of the original game or the GPL engine, unless you went and signed a different license with them including if you wanted to sell anything derived from their work.