RE: 4 stroke .81 size Os or Saito?
Personally I think there equal in quality of mechanical performance, Saito has OS crushed in the price per HP category and usually power to weight category thats why I use them.
Id put a Saito 100 on it, power to weight its an awsome motor. Maybe a little excessive but I have yet to hear " I don't like this plane it has too much power"