How to transplant a 20-yr old peach tree?

My then 4-yr old iduously nurtured a wretch of a dying sapling and pulled it from the brink of tree heaven. Apparently perseverence also works in the corporate world, and It's fitting for his coming children to have this memento in their own garden. Logistics dictate much of the tree's crown be cut. How much is too much, and when, please? Many Thanks!