You need to decide what you want, first. 80 MPH is only going to happen with a serious race kart (bring your checkbook). In fact, 60 isn't likely to happen without a good budget. In both cases, they will need to be full-on race karts. Tall tires and race karts is not happening, either. If you want tall tires, look towards a fun/yard/offroad kart or buggy. The combination of noob fabrication and high speeds is a guaranteed date with your maker. When your @ss is on the line, there's no room for booger welds and duct tape.
Now- to put things into perspective, 40-45MPH is faster than you think it is. With your butt only a few inches off the ground, things seem faster than they are. It is also a much more realistic target.
Since I'm forced to assume that you've never fabricated a vehicle, my advice is this: Shop around for a kart that meets your desires, whether it's complete and needs a bit of wrenching, or no engine, or whatever. Fix it up, have fun, and get intimately familiar with the workings of a machine. Start modifying it, changing things, and learning what you can do. Eventually, you can work toward building your own that will be your "ideal" machine.