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.
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.
Not sure if this goes here but I do have a question I hope you can answer. Ok so I'm trying to download a mod that is not from lfs, its a zip file and I extracted it and put the data into the LFS data folder for the game, I open it and all I see is this. What did I do wrong. Ill also send the mod data over, I'm just not sure how to upload it.
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.
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.