I’ve found a good key combination to work in many games including Neo Geo.

Here’s the PiTFT mapping :

• (5) TAB : Insert coin for Neo Geo games
• (6) ENTER : Start
• (12) SPACE : Select
• (13) X : X
• (16) Y : Y

You’ll find my Adafruit-Retrogame version here : https://github.com/paulgreg/Adafruit-Retrogame/tree/2buttons