I hope others chime in here too. Drift boats are not ideal for motors but it works. If they had less rocker and more width at the rear then then it would be different story under power, but they'd be not much good on the river. Then they'd dig down at the transom less and rear up at the front less when under power.
Many will tell you not to use more than 10hp on a drift boat. I have a 20hp Merc and use it all the time. Just be careful. Don't turn sharply under full power. I have a 4' foot long piece of white PVC water pipe I camp onto the throttle handle of my old outboard so I can still sit in the middle of the boat and work the motor. I makes you wish you had a 3' foot arm when trying to turn sharply. But usually it's just what you want.
If you don't put a guy behind the rower you can put a motor well behind the rower but a foot or two forward of the transom. Then you could run 60hp. But two passengers, when you had them, would have to sit side-by-side on the front seat. Which is what not-fly-fishermen usually do anyway.
If a guy figured out how to put some sort of hydraulic or even mechanical trim tabs at the rear end all that "digs down at the rear when under power" stuff would go away.