The database doesn't work like that. There is no 'administrstors' table. There is a 'mdl_user' table that holds all the basic info for all your users. Each user has an 'id' in this table. This is cross referenced with the 'mdl_role_assignment' table. Which sets the role assignments for users, including the admin role. Therefore you must first search the mdl_user table for a replacement 'admin' user and find the id. Once this has been done then you can add a new record to the 'mdl_role_assignment' table and manually give this user admin rights.
For example, if I wanted to give the user with id 20 administration rights.
INSERT INTO `mdl_role_assignments` ( `id` , `roleid` , `contextid` , `userid` , `hidden` , `timestart` , `timeend` , `timemodified` , `modifierid` , `enrol` , `sortorder` )
LAST_INSERT_ID( ) , '1', '1', '20', '0', '0', '0', UNIX_TIMESTAMP( NOW () ) , '0', 'manual', '0'