I checked the Ampeer Plane ratings section. There were three Four Stars listed. My guess is that they were the "40" size based on the under six lbs weight. Two were on Astro 25Gs with 16 cells; one was using a MaxNeo on 20. Going by that I'd say an Astro 40G on 20 cells or a MaxNeo on 25 with 4/1 gearing would get you a nice flying bird.

Breaking that down into more universal terms look for a 24 to 30 volt motor that can pull 25 to 35 amps.
Walt