I would recommend the Beta. It is basically a marinized Kubota. Millions of Kubotas were put into tractors all over the world, so parts are widely available and reasonably priced.
I Based on the displacement of the P323, I think the 25 should be fine. Rule of thumb is 1 HP per every 500 lbs, so at 12,800 lbs, this works out to about 25 1/2 HP.
You would be unhappy with the performance of the 20, and the 28 (is it still available, or has it been replaced by the 30?) would give you some extra margin for foul weather and current.
https://www.betamarinenc.com/all-engines/