Found the source, here you go:
http://files.frashii.com/~sp00nz/Doom/files/ports/nxDoom_Beta2_R2_Src.zip
----
EDIT
--------
Also, I know this isn't probably what's on your mind atm, but when you port SRB2 to the XBOX, do you plan on having a MOD launcher for it too?
The reason I ask, is:
1...