I thought "bugfixing" at first, but then I realised that you said "under a liberal license". A liberal license would allow people to do more with the source - they could use the JJ2 engine for their own games, much like what's done with the Quake and Unreal engines. That does assume that the source lends itself to such - it could be a wonderful example of flexible code with a cleanly-defined separation between game-specific and game-independent modules, or it could be a mess of hardcoded spaghetti logic.