Whether or not you can get a rental car over, you will not have time to really see Tikal in one day if you drive yourself. If you hook up with a driver, you will still have little time, but at least you won't have to worry about taking a wrong turn. Tikal is a big place. Give yourself a couple days, and try to stay close to the site, if you can. So nice to wake up right outside of it, before everyone gets there. If it has to be a one day trip, better to fly, and get in as early as you can. It gets really hot, and there is much to see. Read up on it first so you know what is important to you to see.