If you play Time, you can get Celebi by holding Secret Slab/Mystery Part and go to 10F of Mystifying Forest. No need for a code.
If you want Mewtwo... Here's the code for a mission that will unlock Mewtwo once you complete it.
122af2e0 00000104
122af2e2 00000001
122af2e4 00000201
122af2e8 00000001
122af2ec 00000019
122af2ee 00000096
122af2f2 00000001
122af2f4 00000001
222af2f8 00000000