It could have a fungal disease? Cut off the dead branches and see if they have dark rings inside. Anyway, take some cuttings from the healthy bits and root a new plant. But put it somewhere new. I don't know what their life span is, but 20 years was good value!

Our oldest choisya ternata bush is much the same. I reckon it is about 20 odd years old. It did have its roots loosened in a gale some time ago, and has not been as vigourous since....thank goodness. It seems to get the occasional die back, which you speak of. None at the moment. Our bush gets a severe haircut anyway from time to time anyway. We have no room for a bush of such dimensions as your chiosya.