The actual differences are not that much. It's not commonly done to put heli engines on planes because heli engines cost more. The carburetor setup on some of them (not all) has a finer midrange adjustment because heli engines spend so much time at part throttle. That adds some cost, along with the bigger cooling head. The benefit of a heli engine is that they are made to run well on 30% fuel and they are ported to top out around 18,000 rpm so the power to weight ratio is better if you're looking for top performance in a speedy plane. Most plank flyers just move up to the next displacement for that, but using a heli engine is an option to get some more power without adding more weight.