Ok I need an engine that haul preferrably around 500lbs as fast as possible, and preferrably under $800. Secondly how do you suggest I hook up the engine? (Should I just make a direct chain link from the engine to the rear axle?)
Used motorcycle, quad, or sled engine- source a whole machine for $800 or less, get extra goodies like brakes off of it. If it's a bike or quad, a straight chain run to the rear axle is fine. For a sled, use the CVT it came with.
a motorbike of quad engine. Most quads come with a reverse gearbox aswell as 3/4 foward. You can then make up a shifter handle, and you have an engine for a shifter kart.
Get an old golf cart. It'll haul 500 easily right after purchase without any work. And if you score one pretty cheap you have some money for a motor upgrade.