VP959, two points:
1. To fly in controlled airspace (where it most certainly is allowed to fly), the paramotor in question needs a transponder. Might be a daft thing to do if ATS where you fly wouldn't allow it in anyway ... But most people in the world do not fly in the UK...
2. The point of my post was that if a paramotor can fit it, so can anything else. Not sure exactly what transponder he uses, but it's not the same type of ancient, power hungry kit you'd find in an average 70's vintage PA28.
Seems to me having or not having a transponder is a cost issue complicated by British aviation regulations, not a practical issue of power and weight...