I don't think I'd replace it... looks like a virus. I'd remove tree, and take my time finding something unrelated.
Anything pear, apple, quince, loquat, mayhaw, hawthorn... related is likely to suffer the same fate... I'd look around at California buckeye or oak or something.
There are so many trees that will grow well in SF... I can't imagine wasting space on invasives like callery pears!
