If you find a car with a very low engine quality < 15% and repair it to 100% using engine parts, you will not be able to start the car. This also assumes you have gas and a good battery in the car. The Java code appears to be using the vehicle.engineQuality attribute to determine engine state but any repairs you do only effect the engine part quality, a separate attribute on the engine part. I would expect vehicle.engineQuality to update as repairs are done or the Java code to use the engine part quality.
Thought I would take a look at a simple clothing retexture mod. I can work out most everything but the last couple steps to keep it from being a hack job. Only a few new steps here from a standard item mod. Create the item definition: <User Folder>\Zomboid\mods\<MyMod>\media\clothing\<MyMod>.txt Create a texture image for the 3D character model: <User Folder>\Zomboid\mods\<MyMod>\media\textures\Clothes\<subfolder>\<itemName>.png Create an inventory icon 32x32: <User Folder>\Zomboid\mods\<MyMod>\media\text