Figured it out: In classes/Product.php change:
SELECT p.`id_product`, pl.`name`, pl.`link_rewrite`, p.`weight`, p.`active`, p.`ecotax`, i.`id_image`, p.`reference`, il.`legend`, m.`name` AS manufacturer_name, tl.`name` AS tax_name
to:
SELECT p.`id_product`, pl.`name`, pl.`link_rewrite`, p.`weight`, p.`ean13`, p.`active`, p.`ecotax`, i.`id_image`, p.`reference`, il.`legend`, m.`name` AS manufacturer_name, tl.`name` AS tax_name
and add to the "WHERE" line:
OR p.`ean13` LIKE \'%'.pSQL($query).'%\'
Then just edit the AdminSearch.php file to display the ean13 field in the "fieldsDisplay" and the "Display products" areas.