Have you tried recreating the control file? do an alter database backup controlfile to trace; Find the file that was created in you bdump folder and edit it. Take out all of the comments and references to your undo tablespace and then run that against the database as sys. But make sure that you set UNDO_MANAGEMENT = manual;

This should remove the missing tablespace from your database, you should then be able to open the database and recreate the undo tablespace and change UNDO_MANAGEMENT = auto;