Well, I'd get some Polyfilla (or any kind of const. cement), fill in the 'hole' and let dry well over a few days. Then I'd sand it as smooth as possible, making sure it's level with surrounding stuff, paint it to match (even acrylic craft paints would do) and then use a good non-yellowing sealer or varnish over it... a few coats each left to dry o-nite, and there you go! It may not be perfect, but then who else is going to be looking that hard at that little piece of one tile but you?

Have you checked with the manufacter to see if you could get a tile from them? Even if the store doesn't carry it anymore the maker of the tiles might still have some.

This product is made by a very reputable manufacturer; Forbo. Have you tried getting in touch with them to see what they might suggest? Maybe visit their website and see if you can get a phone number for customer service. Customer service may put you in touch with their claims division who might be able to guide you throught some type of repair or other suggestion.