Small engines can be purchased for many self hobby types. Places like Northern Tool are great for small engines anywhere between one horsepower to 23 horsepower engines. These engines are purchased for many projects like water pumps, go carts, homemade generators, log splitters and various other projects. Pay attention to your project before purchasing an engine. Some projects require verticle shaft engines where others need horizontal shaft engines. The engine configuration generally will not interchange with the others. You will probably need accessories also to couple the engine to whatever drive system you use. Generally the same store, such as Northern Tools, will have sprockets, and drive shaft coupling parts to connect to whatever parts and components that need to be driven. If you plan to make a go cart briggs and stratton and tecumseh both have race cart series. It would make sense to use the same engine so that you have all the same aftermarket parts available to you. If you plan to build a generator make sure the engine has a good governing system to control the engine rpms. A generator engine should be run around 3600 rpms to keep the cycles at 60 hertz as regular ac power.