I'd go with the OS .72 FS alpha. GREAT engine and probably give you as much power as the Saito .82 (which is also a great engine). If you think you would ever consider putting floats on the plane go with the larger displacement. I've had that plane for 8 years and she flew well on an old OS .70 Surpass but when I put her on floats I moved up to the .91 Surpass. But then I'm at 5300' elevation.

In any event, stick with a 4-stroke from OS or Saito and enjoy your Cub.

Augie