Most chain saws like to run at high RPM. If you let off the throttle while cutting they bog down, meaning they don't have a lot of torque. On a kart that means poor acceleration, or gearing it down for better acceleration and lower top speed.
Common consumer grade saws will be in the 33 cc range, which is pretty wimpy for a kart with 4 wheels on the ground. It may work for a two wheeler on pavement, but I'd rather see something with at least 45cc for a 4 wheeler. The disadvantage to that is that the larger chainsaws are usually treasured possessions of their owners. I sure wouldn't part with my 55 cc Husqvarna..... although I have to admit that I always think of how nice it would be to have a kart or bike that starts up on the first pull like it does when I'm cutting wood. (I love that chainsaw!)
Your other option may be to get two cheap consumer saw engines and mount one on each rear wheel. I saw a youtube video recently where a guy has a twin Homelite kart. It seemed to get along pretty well. Believe it or not, you probably could mount two different but similar engines on the back of a kart without experiencing any difficulties at all. I knew a guy who had a twin engine RC plane with dissimilar engines, and it didn't seem to make a difference. I have a go kart that drives straight even though one back wheel is powered and the other isn't. Putting a Poulan 45cc on one side and a Homelite 33cc on the other would surely be better than either one alone. But get two the same if you can find them.
