GP2X MAME Port
You will need to get a copy of the ROM files from elsewhere and place them in the roms/ directory of each distribution.
Other GP2X MAME ports
Building MAME for the GP2X yourself
Some small changes to the xmame source code are in this patch.
This can be applied to the MAME 0.102 source. As MAME is too big to build all in one go for the GP2X, this version has been configured to provide support for just one machine - the Williams hardware that powers games such as Defender and Robotron (see movie).
The patch currently uses SDL as scaffolding, but draws straight to the Linux framebuffer.
For information about how to build MAME for the GP2X from source see the wiki page MAME_Development. This documents what you need to modify in order to build other parts of MAME of your choosing. For emulations of older hardware this should be quite simple.