You need to unpack the data.acd file for the car with QuickBMS, google it and download the program and the Assetto Corsa DATA.ACD extractor script for it. There are some great YouTube videos on how to use it, watch those first so you don't get confused as it's not that user friendly.
Make sure you create a folder in the QuickBMS folder with "exactly" the same name as the car folder eg: ks_ferrari_488_gtb and put a copy of the data.acd file for that car in that folder, then create a folder inside the QuickBMS ks_ferrari_488_gtb folder called: data
Unpack the data.acd file into the data folder, if you don't do it this way and have the same folder name as the folder it came from it will get scrambled. Put the data folder full of the files into the actual ks_ferrari_488_gtb car folder in your game directory, rename the original data.acd file in that directory to something else, data.acd.Orig for example or the game will ignore the data folder and use the data.acd instead. The game will run fine using the data folder, you can then look into and modify with notepad all the files in there, be careful to keep copies of the original files as if you mess something up the car may not load, I would start with the car.ini file for the weight, it's under "TOTALMASS="
Hope this helps.
