Hello Nick
The first point I would make is that the boat is "plated" for RCD classs C with a stated maximum weight of engine of 25Kg.
Having said that many (most?) boats have engines which exceed this figure if only marginally. Indeed, when I bought my boat Honnor Marine were recommending a 5HP Honda as the perfect match, it weighs 27 Kg.
I do know of a couple of CC19s (Zimp and Falcon) that are (or have been) fitted with the early Honda 6 Hp motor, which was twin cylinder, and I think tipped the scales at 37Kg. These engines are beautifully smooth and the boat seems to cope with the weight.
These larger engines usually have larger props which are great for pushing the boat through the water, but they generate more drag when sailing, swings and roundabouts I suppose!
Trailing the boat with a heavier motor mounted on the boat, will put considerable strain on the mounting bracket on rough road surfaces, You may also find it difficult to achieve sufficient nose weight on the trailer for stable towing. Although this can be addressed by moving the trailer axle further aft on the trailer, as I have done with my HM trailer. But that is a different matter.
Finally, 37 Kg is quite a weight to be lifting in and out of the boat when you need to remove/replace the engine. I find 27 Kg enough of a struggle.
I like the idea of an electric start, but in reality I do not need it, my Honda 5 starts second pull when cold and first pull when warm.