A friend of mine reworked a 23' Seacraft....
He installed a 150 gallon fuel tank which is what I would put in mine too if I was reworking one. When installing it, I would try to push the fuel tank as far forward as will be allowed by the boat. Also, to accomodate the heavier 4-stroke engines you will need to raise the floor at least 2" to accomodate for the self-bailing feature of the boat.
As far as engine, I would go with either a Suzuki 300 or a Yamaha F300. I personally am fixing to pick up a 23' Seacraft thats already bracketed and enclosed transom and I plan on putting a new Yamaha F300 on it. My friend with the above boat installed a Mercury Verado 350 SCI on his. Said that it works, but it is really too heavy for the boat. Said that if he were to do it over again he would have installed a Yamaha F300 on it.