If you look closely at the engine and compare to Zenoah 23, 24, 26cc and even larger, I think you'll see where the engine comes from. They can make BIG power and turn a ton of rpm.
After making an ID of where the engine probably comes from, the specs of 25cc and 3.5 hp have possibly fudged. To my knowledge, a 25cc doesn't exist. Of course, neither does an 80cc China Girl.
http://www.davesmotors.com/s.nl/c.885035/sc.12/category.27867/.f