I have a Tohatsu 3.5hp on my 5.3m Humber. She is quite heavily laden with kit for diving but does 4-4.5knts on the littl'un. This is enough to get me home even if it does take a while.
Fitting larger engines seems pointless, if the main packs up you are just wanting to get home, if it packs up regularly then you need to fix it or replace it. The smallest most compact engine I could get was what I looked for, it is hopefully never going to be used!