These systems feature an upgraded CPU with four cores clocked at 804MHz and 256MB of RAM. Because of this extra power, a New 3DS can emulate almost the entire Neo Geo library at full speed (60 frames per second) with high-quality audio, minimal frame skips, and advanced rendering options. The Old Nintendo 3DS / 2DS (Original)
: You can use tools like the Ultimate 3DS VC Injector to "inject" a Neo Geo ROM into a CIA file. This creates a standalone application for a specific game that appears on your Home Menu.
Set your aspect ratio. Neo Geo natively displays in 4:3. You can choose to stretch it to fill the 3DS screen or keep it pixel-perfect with black bars.