There is currently a Heretic community project going on over at Doomworld that is well on its way to replacing all 5 episodes' worth of levels. Obviously, all the individual contributors would need to give permission, but the project head (who also contributed a level) is interested in helping. Even if only half of the participants GPLed their work (for instance), that would be a massive boon to this kind of project. 45 levels is a lot to replace! Link:
http://www.doomworld.com/vb/wads-mods/43148-heretic-treasure-chest-still-need-e4-and-e5-short-list-of-abandonment-added/