Today my new Nissan 4-stroke outboard arrived and to my relief I found that it fit perfectly into the motor mount built into the Tiki 21 bridgedeck by the original builder. This mount is lower than deck level, so the 20" shaft gets the prop well below the waterline and as you can see in the photographs, it does not protrude high above the deck level.
The Nissan/Tohatsu line of outboards come in a 4, 5 and 6 horsepower size, all three of which are the same weight: 55lb.s. I chose the 5 because this size comes with an external fuel tank and line. As I found out long ago on my Hitia 17 with it's 3.3 hp Evinrude, there is nothing more inconvenient than trying to fill a small internal fuel tank when it runs dry in choppy conditions and you end up spilling gasoline all over the engine and the decks. I also wanted forward, neutral and reverse gears, which this engine has, and the quiet, reliability, and fuel economy of a four-stroke.
I found the best price for this engine at www.onlineoutboards.com. I ordered it on Monday and it arrived today, shipped for free via FedEx.