I replaced a Vire 7 with a BMW 7 HP
diesel. The BMW D7 was a marinised German "Hatz"
engine. I did the conversion in the 80's on my Australian fibreglass production Tophat 25. I had to get a new prop to match the
pitch and rotation but it gave me all the
power I needed.
I wouldn't consider any engine bigger than a 12 hp at the most and I think even that is too big. Diesel engines need to be loaded up or the bores will glaze.
Work out your waterline length and
hull speed then consult some engine manufacturers and agents and see what they recommend.
I hope my
advice helps. I have no idea if those engines above are still available.
Regards, Wayne