Goppier and @UndeadxReality have released a set of custom mystery gift event distributions for the Gen 3 Pokemon games on the GBA. The attention to detail of these distributions is really impressive.
The following mystery gift events are capable of been distributed:
Old Sea Map
FireRed and LeafGreen:
All 6 Wish Egg Pokémon (Chansey, Drowzee, Exeggcute, Farfetch'd, Kangaskhan, and Lickitung)
Head over to the release thread: Custom Mystery Gift Events GBA to learn more and try these distributions out for yourself.