solder the wires from the micro switches to the contacts on the gba motherboard, probably harder to connect a monitor to the gba mobo.
good luck with the project. the original mario vs donkey kong would be great to put in a real nintendo cab.
I had a crazy idea about putting the guts of a ds into a dual monitor playchoice cab for new smb, but there just was no good solution for the touchscreen with the pc10 bezel.