For me the noise and vibration would be unbearable for that B&S outboard. Think snow blower.

The typical outboard uses the water not only to cool but to also help quiet the exhaust. My advice is to look at this in a slightly different way. Not as an
emergency backup that you may not use but as a motor for the dinghy that you will
use on a regular basis. That will be it's
primary purpose so it needs to be weight and size appropriate. If you can leave the motor on the dinghy while towing it you may be able to get something a bit larger....say 4-5 hp. If you have to carry the motor clamped to the stern rail you may need something smaller...2-3 hp. The type of dinghy will also determine what size motor you can go with. If you do have any main engine problems then you can lash the dinghy to the stern quarter and use it as a push boat...that may be the most effective way since it will probably be a short shaft motor. It does not need to be big enough to push you upstream for 50 miles...just enough to get you out of the way of an oncoming barge

or to the next town, lock, or suitable anchorage to effect repairs.