20 - 25hp!!!! Firstly, the designer will have determined a maximum practical engine size for the boat, and will have engineered the transom accordingly. It is unlikely to exceed around 15hp, and could be dangerously overstressed if a 25 is hung on it and run hard, partiucularly in a seaway. Check with the builders if they are still around.

This of course assumes the hull is not designed to plane under power. If it is, then it will take a much larger engine, and be much more strongly constructed. But the sheer weight of even a 25hp+ engine could have very unpleasant effects on handling under sail.

A 20 foot daysailer in calm conditions should go quite adequately with around 4-5hp. Double that will keep the boat going against pretty well anything you meet. If you really insist on going out when conditions are at their worst, then possibly 15hp (with the extra weight and fuel consumption to take in to account as a minus) might just be justifiable.

Anything more will just be a waste of time, money and fuel on a 20 footer, simply making larger and larger wash as the hull digs deeper into the water (and your pocket!) . A displacement hull can only go up to a certain speed through the water, dependent on the waterline length, and doubling the power will give very little extra speed, but at 3 - 4 times the fuel cost! And in a 20 footer you will likely get very very wet anyway as you try!