Not too easy to say as I dont know which files you have modified at all.
Maybe this could work:
1.Create a new project if you havent (in the Asset Browser)
2.Copy the prefabs of the vehicles that you modified in this project (choose copy without generatting a new GlobalAssetId) [AssetBrowser->RightClick on the file -> "AssetPackage"->"Copy to Asset Package"
3. Right Click on your Project (Asset Browser) -> press "Publish Mod Project"