There are many emulators to choose from and support includes the latest GBA4iOS 8.3, 8.2, 8.1.3 and PS1, PSP, NDS, NES, SNES, GBC and many emulators. GBA4iOS 8.3 being the most popular.
HOW TO INSTALL GBA, PSP, NDS EMULATOR ON IOS 8.3 & 8.2 FREE NO JAILBREAK – GBA4IOS, PPSSPP, NDS4IOS