I converted the Eagle 2 to electric using a Mega motor/gearbox combo . that was several years ago. Now I would use a Scorpion or Hyperion outrunner and 5S pack. Would be more than enough for the Eagle 2.
Whichever you choose you would definitely need to a little more power for floats, but I think that maybe a .60 size outrunner would not be needed. The Scorpion 3032 series is capable of 1065 W using a 5S pack while the Scorpion 4020 series is capable of 1500 W with a 6S pack. The Eagle 2 would not need as much power as the big Cub since there isn't as much drag from the airframe and lighter weight. The real problem is of course the friction from the floats against the water and extra drag. I have not come across any "rule of thumb formula" for float planes is regards to motor size. I guess there's too many random factors involved.
The ability of the motor/battery pack/prop combination in determining the amount of thrust needed to break the floats free from the surface tension of the water is what's at stake here.
My guess for the Cub I would use the Scorpion 4020 series motor. They are very powerful and efficient as well as a higher quality than the E Flight motors. beside you can always ask Lucien at Innov8tive Designs http://www.innov8tivedesigns.com/
He will be happy to answer your questions. You can also go to RC Groups motor thread where Lucien Miller takes all questions.
Good Luck.
I'll be converting a Sig Seniorita ARF to floats for next summer using an old set of Gee Bee ( milk bottle) floats.