No idea how new you want or what your budget is, but have you tried David Anderson in Tayport? Also not sure what model of Humber you have, but Ardoran (just south of Oban) has a Honda 4- stroke 50 if the transom will take it.
Incidentally. What flavour is the 25? If my Merc turns out to be beyond economic repair I might be interested in a twin for my Suz DT25.....
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