Hm... You might be able to repair the board yourself. A common problem with light guns is that the diode's legs can break off from the opto PCB. Check that before you drop what could be a wad of cash in a new opto board... Check for broken wires, too. If I remember correctly, the mechanized attack game has the guns mounted to the panel... Violent movements could have broken a wire.
Good luck!
APf