using a motorbike engine in a kart is harder to do than putting in a small engine, say off a chipper or cement mixer.
its different for every engine and all depends on what kind of kart youre building.
buggy? yard kart? street/race kart? etc?
you need to make more complex engine mounts (small engines bolts on the bottom) and make shifter linkages, fuel pump, throttle cable, clutch cable....
small engines have alot less power to weight ratio than a motorbike engine.