If you have a real do-it-yourself mentality, then you've probably been tempted to approach your go-kart project with the idea of sticking a lawnmower engine into it, or taking a chainsaw apart and using it to power your new wheels. Now, this can be fun, if you know what you're doing, but your best bet is probably going to be actually looking at some real go-kart engines. The engines specifically built for go-karts are more durable, more safe and more likely to give you a superior performance. If you're entering a motorized bathtub race, go ahead and use a lawnmower engine, but for go-karts, a go-kart engine is going to give you a more satisfying ride for two main reasons.
If you've never put an engine together before, it's really not safe to try and custom rig and mount a go-kart engine from something else. To make a comparison, you could say that using a real go-kart engine is like building with Legos, but using a custom-rigged engine is like building the same construction from raw melted plastic. In other words, if you're not building and arranging your kart's engine with a high degree of precision and accuracy, with all parts properly and securely in place, it's likely to come unmounted and create a serious problem on the road.
Even if you mount and customize a lawnmower engine with a high degree of precision, there's still the unavoidable fact that that engine was built to power a grass cutter, not a vehicle. This means that you're not going to get the same control you'd get with a go-kart engine, and you're not going to be able to reach the same speeds. Besides, you're going to get tired of pulling that rip cord every single time you want to take your kart out for a spin. Go shopping for a real engine for your kart, and you'll thank yourself later.