If you're designing for Part 103 you will want to use the AC103-7 appendix calculations for speeds. Not as actual design values, but as a sizing guide. The calculations in the appendix are very conservative, and accepted as proof of 103 compliance without actual demonstration. So you can have a legal ultralight with somewhat higher performance than the regulations would suggest.

As for engines, the Cuyuna is not a bad choice. Lots of them still around, parts readily available and far cheaper than Rotax.