Same for me with my 1964 Merc 6hp. The ONLY thing that worked was a hammer and chisel for removal of the "cover" nut. I had somewhat of a difficult time finding a replacement part...but did.
There is a seller on the USA ebay right now who is selling one.Do a search for mercury outboard tool and you should find it.Depending on how much it goes for it might be cheaper to just to destroy the old one during removal and buy a new one.Nathan