There are and they use legit models made by the mod's author and they ask for permission. But in this case this is not a legit model, its ripped from squir and squir's models are paid, so no you can't use this model.
This is how a mod file should be. If you want to tweak it, you'll need to download the LFS Editor, then you need to drag these files into LFS Editor > Data folder. Once you done with that you need to load the mod by clicking Vehicle editor and load on bottom left corner, then you can click test mod in LFS button as i shown on the screenshot. If everything is right, you should be able to go into LFS single player, go to garage > mods > test and choose the mod.
This rar file is not a mod file, it only contains xrg's setup and an engine sound for xrg, i'm not sure what's the AT file in the tweak folder but this is not how you test another mod in-game beacuse this file is not a mod file.
You can import the mod to LFS Editor and change the engine stats, then you can test it in single player. So you can't change the engine stats without LFS Editor.
You can, however you can't publish it without a permission from the mod author, you can only use that version on single player. If you want to publish your custom version you'll have to ask permission from it's author.