You won’t necessarily lose the lilac. A Montreal (yes, Canada) web site for that city”s botanical gardens (among other sites) advises as follows for TREES: If the central leader is broken, damaged or very weak, or has a dead terminal bud, choose a vigorous lateral branch to replace it. If the best-located lateral branch is growing at an oblique angle, cut back the damaged leader as close to this branch as possible. Place a stake in the ground to support the lateral branch. You will be able to remove the stake after two or three years. If the tree has an almost vertical lateral branch, cut back the damaged leader just above that branch, which will straighten up and eventually take over from the original leader. You can read more here: http://espacepourlavie.ca/en/pruning-deciduous-trees
You won’t necessarily lose the lilac. A Montreal (yes, Canada) web site for that city”s botanical gardens (among other sites) advises as follows for TREES: If the central leader is broken, damaged or very weak, or has a dead terminal bud, choose a vigorous lateral branch to replace it. If the best-located lateral branch is growing at an oblique angle, cut back the damaged leader as close to this branch as possible. Place a stake in the ground to support the lateral branch. You will be able to remove the stake after two or three years. If the tree has an almost vertical lateral branch, cut back the damaged leader just above that branch, which will straighten up and eventually take over from the original leader. You can read more here: http://espacepourlavie.ca/en/pruning-deciduous-trees
We hope this helps—