RE: What size of an engine for my Astro Hog?
Mine (RIP) had an OS 70 Surpass I. It weighed just under 8 pounds. It would take off at 1/2 to 2/3 throttle from a grass field. It would pull a really big round loop and I never thought it was underpowered.
My brother has one with an OS or Magnum 91. It is not overpowered. It has stronger vertical climb than mine had but that's about the only difference.