I would try for at least two days with ‘4 marked fake eggs’ if she stays on the eggs. To determine if she is seriously breeding. Then you can change one fake egg with you're halfway incubated egg (another mark with pencil ✏) .
After the chick has hatched and the mother leaves the nest you should throw away the fake eggs.
Better not wait until the chick has hatched. If the broody hatches the egg, she will accept it. Chances are quit big she wil reject and hurt a chick from the incubator.
Sorry to hear you have only one egg . It can be hard for a chick to be an only child growing up with adults.