A complete Galaga is a poor Mame cabinet choice. A very poor choice. If you happen to live anywhere near St. Louis I will come pick that Galaga up, and drop you off an empty black cabinet and toss in a big VGA monitor to put inside.
Oh, and for the record that cabinet will hold a full streetfighter 2 layout. I have seen plenty of them converted in that manner. Don't extend the control panel, it will look really, really stupid.
Matter of fact, don't do anything to the machine at all. Just post it for sale/trade on the rec.games.video.arcade.marketplace newsgroup. You should have no problem getting someone to come to your house, take that machine away, delivering you an empty conversion cabinet and giving you some money for your troubles.