How to select and electric propulsion system for my experimental Ultralight?
FLÜGEL World Directory Of Light Aviation 2019/2020 - English PDF could be a good starting point. On pages 278 - 281, a total of 71 lines of electric motors from 42 manufacturers are featured. Disclaimer 1: some data fields are incomplete for many of the entries. There are German, English French and Chinese versions in paper and pdf, starting at €8. Here's the paper version in German:
Welt-Index Ultraleicht und Flugzeug 2019/20 Wings of the World

In any version, the aircraft listings themselves may also be of interest, as aircraft models which are available with electric motors as the primary powerplant are highlighted with a green e icon. In browsing those, you may glean some emperical inspiration in terms of electric motor vs aircraft size. Disclaimer 2: the power-plant details for the aircraft listings are not always complete.

The free annual e-flight Journal by the same publisher includes motor manufacturer adverts which may be of some use.

Lastly, at the end of the online Kitplanes 2020 Engine Buyer’s Guide are four electric motors from one manufacturer.

Final Disclaimer: I have no vested interest in these magazines, publisher or any products listed or advertised.