I had a 4hp Yamaha Longshaft on my old 5.25 Viper. I sourced a lower pitch prop for it and managed to get about 4knots sometimes 5. It was enough to get you way from trouble if the main cuts out near some rocks but it would be a long ride home
I kept mine mounted directly on the transom which is the best place for it (don't like the brackets), however do make sure you maintain it as its going to get wet every time you go out even if you dont use it and you will also need to improvise a way to secure it well as the saddle barckets on the smaller engine don't take much of a pounding.
Having an internal fuel tank is handy as when you want an aux its generally going to be in a hurry, so hooking up a fuel tank and trying to measure oil into a jerry can when you are in an F6 200 yards from the breakers is not a great situation to be in
I always made it a habit to run mine at least every other journey to make sure it was going to work when I needed it to.