Hi, Paula,
Did your kitten use her tray before she was neutered? If so, it should not be too difficult to train her to use it again. Does she always defaecate in the same place? If she does, put her tray there for the time being, and if she starts to use it there, move it a little at a time to where you want it.
She will also have left her 'scent' behind, which provokes her to go back to the same place. You may be able to buy a preparation from your pet shop or your vet which will completely remove any lingering aroma which provokes her defaecate in a place where she has been before, but where you wish to discourage her.
Good luck!
Walter