I have treated my mop top for scale before some years ago and have just returned to the house after it was rented. I have cut a lot of the dead branches off. I am on South Australia, do I cut the thicker branches off and spray with scale spray to let the tree start again in spring with new branches?
The wonderful thing about mop tops is their natural shape. Yes, you will need to remove any dead or diseased branches. The problem is that large pruning cuts are prone to rot and the new growth whilst vigorous tends to be vertical and the tree will never regain it rounded shape.
Christopher Bricknell an RHS pruning guru recommends you replace the tree.