Hi Terry.
You really don't need to worry about Torque,all you need is The Recomended Max & Min Power Ratings of Engines in either Horsepower or Kilowatts for the Safe operation of your boat,all boats should have a Compliance Plate with all the above info.
The easiest ways to assertain The Shaft Length is to ask Dealers,check Makers Site for Specs,or simply take the boat to a Dealer & they will measure it up,if they don't already know what fits,then tell you what Engines are suitable,or you can measure it up yourself & shop around.
Good Luck.