1GM10 and Campbell Sailor Prop
We upgraded to a 1GM10 and Campbell Sailor Prop from the SB8 and original prop on our '77 h27. That combination easily cruises at 5.5 knots, and will take us to hull speed on flat water.
We met a member of the Lake Erie Hunter club a few years ago who had upgraded to a 2 cylinder, 15 HP Yanmar on his h27. He was happy with the result, and there was no problem getting the engine in. But he admitted it was more power than he ever used.