This sounds familiar...
I once bought two used lawn mowers for $75. I only got one to run, and the parts from the other one would have cost over $200 to fix up, and then the mower would be worth $150.....it ended up in the trash. That guy just sold me a mower worth $75 for $75, and i took his trash for free....
Maybe you can just get what you NEED to save some space and time. Also, a running go kart could have many issues and expenses that you would find out about after you bought it. The engine is only one piece of the puzzle.