You can't zero fill a sector you can not write to. If sector zero is 'bad' and can not be read from nor written to (because the disk used up all its spares), then you can not make the disk usable. The MBR IS sector zero because your PC BIOS assumes it is sector zero. See: http://www.diydatarecovery.nl/kb_mbr_article.htm
. Since HDD regenarator can not fix sectors it is safe to assume the disk used all its spare sectors.