Not a bad looking boat at all from the pics. It looks to have aluminum heads and electronic ignition, motor looks clean, not all butched together. If it is a 10-1 motor built right, potentially 500 hp is very doable, but no way of knowing without all the specs.
As far as the shifter, jet boats don't normally run a lock-out forward/reverse lever. You use the ability to quickly "shift" between forward and reverse to help steer in slow speed maneuvering . Really hard for it to drop into reverse at speed. The friction in the cable, and thrust of the water out the nozzle will keep it open, plus I know I rest my hand on the handle while driving most of the time.
The brass fuel selector switch usually has a pointer end on the handle you rotate. It should point to the tank it's flowing from. If you have a vacuum pump or just some hose, you can take the lines off the valve and pull a vacuum (or suck through a hose) from the carb side of the valve and hold your finger over the pull side of the valve to verify which side of the valve is open (flowing). Off would be 1/2 way in between, or straight up and down.