You'd probably just be better off buying something like an RLM 31 with twin diesel inboards on outdrives with an engine bay you can stand in, hang on a minute I know where there is just such a fine boat for sale.... Oh yes it's mine!
Because I am a self taught engineer it was easy for me to fit. I was lucky and found the engine brand new on ebay for a 1/4 of its new price and the boat already had an enfield leg.How much it has cost me I don't have a price You need to keep looking for a suitable boat. I found a norman 25 with a perkins diesol engine in it which is about 50 HP Although it does need a refit.
1. If the engine is oil injection then they run better at low revs than pre-mix engines 2. You must have a second power source in any tidal waterway, on a norman 20 that means a secong engine 3. Get the biggest 4 stroke outboard you can afford/fit/the bracket will take, and use that most of the time. In open water its not a big hassel to control the engine from over the back of the transom 4. As has been said check the strength of the transom.