It wasn't written from the ground up but built over the original (copyrighted) SNK system ROM code and data. Therefore, distributing and especially selling it is theoretically illegal unless you have a legal right to do so.
Who cares, right ?
If you consider copyrights and open-source projects, check out Neopen BIOS.
Difference with SP-S2:
- Cheat database with in-game menu
- Jukebox function
- Error handling