Mitsuda did in fact compose the main theme for the original version of the game World Destruction (
Shunsuke Tsuchiya rewrote it for the overseas version), as well as several other pieces (他). This is confirmed on the sampler album for the game:
http://vgmdb.net/album/9371. There was no full OST, so I'm not sure it's known exactly how much music he wrote.
This album is for the anime though!
