To turn it by hand just go to the end of the motor (picture four) and grab that big plate and use it....but by just looking at the spark plugs you may have some corrosion on the cylinder walls too, making it hard for the engine to turn over. I would get some Seadoo oil or fogging oil and put some in the spark plug holes, let it soak, try and turn it after that with spark plugs out and by hand. If your starter works you can just tap the start button and see it the extra torque will turn it over too, caution this could mess something up if the pistons are stuck with rust/dirt in the cylinder walls. Disconnecting the pump will also help, because you don't know the condition of it.
Three other things.. you can take the rave valves out and use a flash light to see what the cylinders walls/pistons look like (somewhat) or if you have a borescope you could put it in the spark plug hole and see what you can see. Lastly, if you are mechanically inclined you could take the head off and do a thorough inspection, but you would need a new head gasket and small stuff for reassembly.
Good luck, lets us know what you find.
