I'm running a 5 HP Honda and I am quite happy with it. No more 2 stroke stink and its smooth and quiet. In fact the only thing that is smoother and quieter is my 50 HP honda main motor.
I am running a 15hp 4 stroke Yamaha which is more hp than I need but the free price tag was definitely right. When the time comes that this has to returned to its rightful owner I am looking into either a Johnson/Evinrude (made by Suzuki) or a Honda, probably 8hp. After running this solid for the last two weeks it runs great just not as smooth as I would like. Also I will go to electric start with a charging system and the electronic throttle control. I found I had difficulty getting dialed in exactly with the hand throttle. Just my 2 cents.