I like the Rotax 912 ULS. Don't own one but my brother does. It has about 250 hours on it and has been flying since 2006. We already had to change the engine mount rubbers. Engine got to flopping around particularly when it shut down. I don't like the mount on his Skyranger but the engine is fine. Zero oil leaks.
My preference is the Lycoming O-235 IF you can handle the 240# dry weight. It has a 2400 TBO. I am building a Pietenpol Aircamper on the cheap. I have flown small Continentals more than Lycomings. The Continentals on the used market are more apt to have 6000 hour TT on them. I wanted an O-200 Continental because of the weight(200# versus 240#) but found in my price range they were bad worn.I bought a Lycoming O-235 with 2000 hours on it. Did a major on it pretty cheap because it was low time. Another thing I like about the O-235 is it is not nearly as prone to have carb ice with the carb bolted to the oil sump.