Oral, tongue, fingers. Rub the penis against her genitals. Ideally, half of the penis will enter, but it won't be as enjoyable as it was originally.
Assuming the dog will love it and allow it, since it is a mutual activity..
It is very problematic to have sex with a spayed female, on the one hand, because she does not feel like it, since the hormones that would encourage her to do this pleasure activity have disappeared. On the other hand, the part of the vagina is sutured where the muscles of the vagina can surround the human penis in the most enjoyable and stimulating way.
The original sex is 10 points (fuck, no, 1000 point), after neutering 3.