Believe it or not a 60 odd cc chainsaw engine will propel a kart at a fair speed. Depending on what speed you want to achieve. I'd estimate a speed of around 30mph been achieved. Although it may only be 63cc it runs very fast around 6000rpm, and is nearly always two-stroke giving it a bit of an advantage. It is fairly easy to adapt to drive a kart. It even has a centrifugal clutch believe it or not that will be very useful in driving a kart. All one has to do is get an ordinary fitting chain and a big sprocket of a racing kart? (maybe). Well anyways it has to be fairly big. Somewhere in the region of 60 teeth for the correct ratio. For ways to connect a simple engine like this up to a gearbox got to this link. Here
Here is a picture of a typical chainsaw engine suitable for propelling a kart.
It is a case of the bigger the better.
In the next picture you can see the automatic centrifugal clutch and driving teeth for the cutting chain. It is just a case of welding a motorcycle sprocket (around 15 teeth) onto the clutch housing/ driving teeth for cutting chain. The ideal set-up is in the picture where the chain arm is removed.
If you build a nice light kart, then you will be going places, expect speed lags with heavy karts and small engines below 55cc.