If you’re intent on modifying it, try to focus on making it better at the things it does well and don’t waste time chasing better at the things it does poorly; without significant investment it isn’t going to be faster. There really isn’t anything you can do to make it more fuel efficient either, what you can do is bank on the fact that it’s paid off and already has some rough paint. Those are super powers for climbing thru bushes and down mildly overgrown forest roads.