The 503 is a good engine. Not made any more, but readily available used or freshly overhauled. But an aircraft with a 503 will likely be overweight and/or too fast to be a legal ultralight, a 447 (or Cuyuna) might be a better choice. But you need to work out the basic design before selecting an engine; successful ultralights are flying on as little as 25HP, half of what a 503 makes and much less than half the weight. What type of configuration did you have in mind?