Surprisingly an ASP 400 (5-cyl) has the smallest outer diameter of the bigger radials with a good enough fill volume (220mm dia). I just dont know about the quality of ASP. Their carburetors are almost useless so a good OS or Saito carb would be mandatory purchase for that engine. It would also look good to equip the Hughes with a high pitch 4-blade prop eventhoug a fourblader is not "correct". That would allow the user to not spin the engine too fast. The lower the rpms in a radial the better it sounds when it needs to use the torque. Prop sound would tone down and aid the fuselage whistling also. Good place for some serious goosebumps