Look for possible clues for it to be late leafing up this year, has it become over shadowed by other trees, or subjected to unusual weather conditions ?
It should be safe to prune by reducing the crown, but I would wait for a frost free period in the Autumn when the tree is in a more dormant state. If you don't feel confident about pruning it yourself, get a qualified arborist to take a look at it.
If you do decide on a replacement, be sure to remove all of the stump and root of the present tree.