5.0 does not contain any mechanics for economic growth or large-scale repairing.
The DMG has a section called "Building a Stronghold" which contains gold costs for various structures (p128 in my version). This attributes gold costs for various structures. You can determine the cost of the full structure, translate the damage done to a town to gold cost per building, and use these tables to assist in calculating reconstruction effort. Most of those put a value on the size of the structure, the cost of the raw materials needed to build it, and the manpower required to construct it. So, a farm would be much cheaper and quicker to create than a fort although the farm is unlisted.
These efforts can be modified by spellcasters. Smaller repairs can be done using the Mending spell, while larger might be done with Fabricate. This would greatly reduce the time needed for reconstruction.
1 minute cast time, 2 lodestones as materials
This spell repairs a single break or tear in an object you touch, such as a broken chain link, two halves of a broken key, a torn cloak, or a leaking wineskin. As long as the break or tear is no larger than 1 foot in any dimension, you mend it, leaving no trace of the former damage.
This spell can physically repair a magic item or construct, but the spell can't restore magic to such an object.
10 minute cast time, consume raw material as per below effect
You convert raw materials into products of the same material. For example, you can fabricate a wooden bridge from a clump of trees, a rope from a patch of hemp, and clothes from flax or wool.
Choose raw materials that you can see within range. You can fabricate a Large or smaller object (contained within a 10-foot cube, or eight connected 5-foot cubes), given a sufficient quantity of raw material. If you are working with metal, stone, or another mineral substance, however, the fabricated object can be no larger than Medium (contained within a single 5-foot cube). The quality of objects made by the spell is commensurate with the quality of the raw materials.
Creatures or magic items can't be created or transmuted by this spell. You also can't use it to create items that ordinarily require a high degree of craftsmanship, such as jewelry, weapons, glass, or armor, unless you have proficiency with the type of artisan's tools used to craft such objects.
Player characters with high strength stats would also be able to help rebuild much faster by moving the raw components where they need to go, and helping craft the necessary components to aid in reconstruction.
These are ways for you to create a plan for reconstruction and how to expedite it if the characters wish to contribute their time and effort.