Is there any of the front section of fin left ? (ie sticking out/they sometimes break accross hole )If there is its possible to reshape and make new hole hole futher back. If not ??? Well if there is any "overhang" to rear of fin it is possible to move the pin that locates fin in fin box to front of fin and put bolt hole to rear. (ie Pin doesn`t need any overhang) You have to be very careful placing pin in correct place (depth wise). Make sure its exactly same depth from base of fin in new place as it is in "old". But if you put pin to front next time you run aground the box will give; not your fin !!!
If neither of above are possible the little device mentioned by Ola is your only choice. Concrete Wave used to make them but try Gun Sails (UK, Brian) ,he`s very good at finding those little bits we used to take for granted. They are not easy to fit but do work fine.
PS Try Ebay for a new "second hand" fin. It might work out cheaper !!!