How big of a go-kart could I build?

Remove the trans axle, invert the tractor, install a safety harness for the seat, attach your gas powered weed eater as a rear stabilizer propeller, and you've got a helicopter. Of course, getting in it to take off can be difficult as it's upside down and landings can be painful, but the shortcuts you can take in getting places makes it all worthwhile. Special note to the really stupid: Don't try this at home - or any place else.

I would make a hummer look alike and paint it cammo...either that or a mobile pool.

A 15.5 Horse engine is a pretty good size for a go-cart. How big is relative to the weight of the material you are planning on using. If you want maximum speed stick with one seat and minimal materials. If you plan on making a vehicle look-a-like with fiberglass stick to a one bar frame, otherwise a dual bar will give you more safety. Use rubber mounts and this will reduce the weight needed for your suspension (a little more of a rougher ride though) and a chain sprocket for your drive. Simply stated, less weight equals greater speed…so make it as big as you want, you’ll just be sacrificing you speed for it.

Shove that bad boy under the hood of a Mini Cooper! Hey, that's what a Mini Cooper looks like; a go-kart with a body! (LOL) That'll be fun removing that Briggs & Stratten engine, and make sure that you cut enough room between the firewall and the fenders!!!!(ROFL) Okay, all fun aside. . . If you have a strong enough frame, and a decent clutch, it might work. You might also want some kind of suspension, add to that, you might want to use the tires from the tractor as well! Have fun.