I have a question. Just how big do you plan on having this robot be? And if you want just a fast robot, I'd recommend just using a T-Maxx chassis and an engine that comes with the RTR version. It also has an E-start so it's easier and more reliable than a pull-start like on other engines. The only thing I'm wondering if you're going to throw in a weedwhacker motor or a chainsaw motor is can the drivetrain parts of a 1/10 scale monstertruck handle it? You might go through a lot of the gears for the 2 speed gearbox that it comes with, unless you're going to make a custom gearbox for it and I say KUDOs to you.
You wouldn't have to modify the chassis much if you're going to use the RTR T-Maxx, maybe just to add a microcontroller to it, because all you would need to control are the servos, one for throttle and one for steering. If you want something bigger to work on, maybe you might want to check out the
Traxxas 1/6 scale Monster Buggy.