The engine in the pic is an engine in a Boston Whaler Jetboat, the 14 foot "Rage" Again, if memory serves me correctly, they use a 701 cc yamaha engine (same as what's in my super jet)
Pump size.....go with smaller diameter, such as Yamaha 144mm. Don't use a 155mm, that's too large for the size engine you are using to turn it. I would go even smaller than a 144mm, but I don't think you can find any bolt-in pumps/ducts that are smaller than 144mm. Using a small engine, you don't want to over-lug it with too large of a pump,or you'll be replacing engines. Think of it like putting a 7" grinding wheel on a 4" grinder. Yeah, it will turn it, but that grinder doesn't really have the mass, or motor, required to be able to turn it for sustained periods, and it will eventually burn out the motor. Same with putting too large of a prop on an outboard, or too large of a pump on a PWC or jet boat.
As for impeller pitch, hard to make a recommendation on the exact pitch you need. However, I would advise using a swirl-type impeller, as they are less prone to cavitation, and they also have a lower tendency to foul in light grass, versus a conventional impeller with its straight leading edges.
Talk with the folks at Impros, this is what they do for a living, they can likely recommend the correct pitch, or even re-pitch your existing impeller.