Ok, wired up 4 x 7805's onto a large heatsink for the 5v from a 12v supply. Tested the voltages when the NeoGeo board is on and voltages are fine.
Using diagram:
Wired ground to pin 1 and 28; 5v to pin 3, 12v to pin 6, audio to 10 and L, red (RGB) to 12, green (RGB) to N, blue (RGB) to 13...