Doug at Hart Aero builds inexpensive U/Ls for sale. For sleeker, more efficient U/Ls, talk to Mark at Earthstar, and look at the ZJ-Viera.
Right, finding a suitable U/L engine is challenging. The single cylinder engines tend to vibrate too much and are generally less reliable because you have to run them so hard. But most of the twins are too heavy to make the weight limit, depending on the airframe.
J-Bird sells Kawasaki snowmobile engine conversions in 440 and 340 cc, that are lighter than the similar Rotax 447 and 377. They partially fill the gap between singles and heavier twins, and have just moderate vibration. I can't recommend the Hirths. Most people have had bad luck with them, including me. The MZ201 is a good engine, but vibrates a lot because of its unusually large 626 cc displacement.
I've been flying legal U/Ls (of my own design) for years, and tried several different engines. Vibration is a pretty big deal. Even rubber mounted, engine vibration tries to tear up your exhaust system, and eventually the airframe. The larger (than 250 cc) displacement singles all vibrate a lot.
And the more you strip an engine down, the more they'll vibrate... for example, if you use a lighter reduction drive, and/or eliminate the electric starter.
The amount of thrust you can get from an engine depends on how much reduction you use to turn a larger diameter prop. You can get excellent thrust out of the Rotax 277 with a 3.0 reduction, for example.