A 2 hp motor would be more than enough, I should think. A raft of that sort won't go much over 5 or 6 mph no matter what motor you put on it. It won't plane, and with too much power it will just dig a hole in the water and scare the ducks.
An electric trolling motor would be excellent. The speed would be pretty close to what you would get out of the gas motor, with alot less fuss. I've used one on a soft-bottomed raft before, and it worked very nicely.
