HI pete
The Saito 72 is really good for the U can do 46. It is not more powerful than a Os 70 and it's less heavy so it should be great. The Saito 72 is really more like a 70. Some people even put a 90 4 stroke on this plane. A 46 fx would be good too, but it won't have the power of the saito 72, so if you wanna have fun, you should chose the saito.
With an 46 fx, you won't be able to pull out of hover like the saito.\
I hope this help
chris