I believe vanilla has gone too far to add this sort of change at this point, but someone might be able to come up with a solution by utilizing scripts and doing
Code:
<command class='update_vehicle' id='<insert vehicle id here>' health='<insert desired health here>' />
The vehicle must not have reached 0% health when calling that with a positive health, I think, it won't become unbroken that way.
As to what mechanic would be used to achieve repairing, it would be up to you. Some examples:
- drop a specific item in vehicle item interaction point to give it more health
- collide the vehicle with a specific hitbox to give it more health.. like a pit stop? or create the hitbox with an item for a different approach
- use a medikit-like weapon to spawn a dummy vehicle or item, just to work as a signal for the script side, to locate vehicles near the event and give it/them more health
- introduce a call that heals all vehicles in a region
- etc..