The biggest chance of having random inboard failure is when you are sailing in rough waters and the fuel tank gets mixed up and debris or water clogs the fuel filter.
Those same conditions are the ones where an outboard is not going to work well on boats any longer than 25' (unless you have a well that moves the outboard closer to the center of the boat). This is especially true of a dinghy outboard that has a short shaft.
I'd spend the effort overhauling the inboard and cleaning the fuel tank.
Having 100lbs hanging off of the stern of an average 28' boat doesn't help it's sailing either.
I thinking of when I get into the harbor, not when I'm sailing in a storm.
The two times I have had engine failure in 26 years of sailing, an outboard would have cured my problem.
One time, I was anchored and had run my battery down and had no way to charge it, and couldn't get my diesel cranked, and had to sail into an unfamiliar small harbor in Pensacola Bay, with the wind blowing into the harbor, and dock under sail. Not as much fun as I like.
The second time, I was coming int the west approach of Nassau, at dark. I cranked my diesel and it ran right up until I hit the entrance and then died (later found to be a blockage in the line as you describe), forcing me to sail to, and drop anchor in, a place I really didn't want to anchor, to unblock it, which took over an hour.
In either of those two cases, an outboard would have worked, and saved me a ton of trouble, which is probably why I think it's a good idea.
It's unconventional thinking I know. But, a lot of good ideas come from that kind of thinking.