I know it's a bit more expensive, but at the time of buying my new rib, I got a new suzuki 4-stroke 4hp aux engine which fits nicely onto the back of the rib.
Knowing that I will be doing some trips locally by myself, I figured it'd be a handy thing to have. At the end of the day, I trust the main engine implicitly, but then, you never can tell I suppose
It's only good for 4.5 knots, but that should be enough to get me out of serious trouble in most cases, or at least help out while I prepare and drop the anchor and call for help.
There have been a few cases of people round here writing off their gearboxes after hitting pots in the river, and they've been saved by their aux engine, which despite slow has always got them home at the end of the day.