I communicated with the company, and they just replied and assured me about the ~85 HP figure.Based on the bore and stroke it is probably based on the Daihatsu 1KR series of engines, such as the 1KR-FE and 1KR-VET. Those engines are used in a variety of models from several manufacturers, though most seem to be from Toyota. (Daihatsu is owned by Toyota.)
What puzzles me is that the Aeromomentum website lists the AM10 as 85 HP at 6000 rpm, but none of the naturally aspirated versions of the 1KR series that I can find on the web come close to matching that horsepower rating. (The 1KR-VET is rated at almost 100 HP, which is more than 85 HP, but that version of the engine is turbocharged.)
Also, some information lists the AM10 at 75 HP at 6000 rpm (which seems almost believable to me, though even that is higher than other naturally aspirated versions of the motor) and some lists it at 85 HP at 6000 rpm (which is hard for me to believe.) I am not aware of any naturally aspirated 1.0L motor that achieves 85 HP. On the other hand, there is a video on youtube which documents ~85 HP for the Aeromomentum engine, so it is all very confusing to me.