Hi!
To be short, I'm currently working on a free reimplementation of EoSD game engine (you'll still need a copy of the game to play).
In the process, I'm writing a "reference" library, that will be used as the base of the reimplementation, and flexible enough to enable modifying the game data.
I've understood most of the game data already, but 100%-compatibility with the EoSD engine (meaning you could make a replay in one engine and play it flawlessly on another) will be hard to achieve, due to the fairly high number of hardcoded game mechanics (who said difficulty/rank?).
As a proof of my understanding, I'll be posting a custom level soon (this will include custom scenery, bullets, enemies and bonus/effects sprites, but no custom players).