I wouldn't mount a 25hp outboard on a 1/4" transom without some serious re-inforcing with thick plywood
and extra grp. The weight of that outboard bouncing up and down in bad weather
would be a real concern. The other problem with mounting an outboard on a transom is that the engine
will be constantly lifted in and out of the water
as you power though the waves. The prop will ventilate and probably need constant attention to get any drive. Finally you are looking at a standard shaft 21" engine. The prop must be well down in the water
and clear of the transom or you loose a huge amount of that drive if the transom blanks the prop.
Personally I would go back to looking at the existing diesel
engine, or find a second hand replacement, particularly on a 37 foot boat. A replacement doesn't need to be the same make or size so lots of possible options. It also means you don't have to carry around large quantities of petrol but can use diesel instead, which I think the forum will agree much safer on a yacht.