Gearing is gonna make a huge difference. I had a mini bike I built with a 5 horse brigs that accellerated me to top speed real quick, but it only did about 25 mph. figure out your power to weight ratio and try to make it as big as possible when expressed as a fraction. weight with fuel and two passengers would be the denomonator and horsepower would be the numerator.
But yeah, you should be able to make a cart scoot pretty quick with a 13 horse engine.