This sounds like an obvious question “How is the go kart going to be powered?”, but it actually is quite important when you are considering, “Who is riding it?” and the question later on down the road, “Where are you riding it?”
Is the go kart going to be:
If the go kart is going to be gravity powered, then considerations of how you are actually going to get the go kart up the hill again need to be answered. Is there a game plan there? If a small child is riding this go kart, then it cannot weigh much more than him, so that they can pull it back up the hill. You want to have fun with this go kart (and speeding down a hill can be fun) but if it is way too heavy to pull back up the hill, it may rarely get used.
If the go kart is pedal powered, make sure that the construction is light enough for whoever is riding it to be able to actually pedal it. Sometimes you can build something that actually is pedal powered, but it is way too heavy and as a result, in order to get the thing to move you have to gear it way down and the go kart doesn’t go very fast. When thinking pedal power, you need to think LIGHT. Light as in PVC piping, or aluminum framing. Again, who is primarily going to be riding this thing: a child, or an adult?
If the go kart is electric powered adequate electrical resources are required. Normally 12 volt batteries will be used, however, they do weigh a lot and consideration for load carrying capabilities of tires, wheels bearings and where to stow the batteries is key. Additionally, just running a circuit to a motor is not going to cut it. Typically sophisticated Electronic Controllers (which are designed to bleed off heat to give variable speed) are needed to run the electric motor. Also a large-duty electric motor will be required to move the whole go kart. Electric go karts will become more and more prevalent as battery technologies increase, but the primary red flags are cost and weight.
Gas engine powered go karts are by far the most popular and have the most readily available parts. Gas engines do however, have elements of danger and concern: IE the engines get hot, they run on gasoline (which is flammable) and they have spinning components even when sitting in an idle position. Gas engines are the most versatile, in that they offer the freedom of riding in all conditions. They do not require charging, subject to fatigue (pedal power, pulling up a hill), and if geared up right can go almost anywhere.
Settling the question about how the go kart is going to be powered then helps settle the other questions that come down the line.