Here's a kart I cut/stretched/reinforced a few years back. Stuck a 16HP engine on it... It'll tote my 260lb butt at an honest 52mph as geared. Engine is stock, but I have a ARC flywheel and rod, a shaved and ported head, cam, springs etc for it. The governor has been removed but the stock valve springs keep it from revving to dangerous levels.
Before, after, and details: