I've had back-up engines on both my boats. First a Tohatsu 3.5hp long-shaft (2-stroke) on a Zodiac Pro, and currently a Mariner 4hp (2-stroke), again long-shaft. I had a Plastimo auxilliary bracket before on the Pro, but never again. Thing use to shake like a bag of tools.
I could probably get by with a standard-shaft auxilliary, but the long-shaft still clears the water if you secure with a ratchet strap and it's under tension.
A Mariner 4hp with internal tank will give you around 45 minutes run-time. You can carry a 4.5 litre (1 gallon) pre-mix spare tank on board. I was looking for a Yammie 4hp that can connect to main fuel tanks - but they're pretty rare.