You got a pretty good haul for the price. I would be careful about how much money you are willing to spend on the project. I bought the donor ski my running 750 BP came from for about $300. You need to investigate the quality of your motor, electronics, etc before you invest a lot of money into such things. I don't feel that a stock 750 motor with a 650 pipe is a huge upgrade over a 650 motor with a 650 pipe, but there are costs involved in getting that 650 running and that should be your concern.
So, with the costs things said...
The motor you have has an 8-stud intake so it is a 650 motor.
It has a keihen carb. I can not tell if that is a 28mm or 38mm carb from the picture. the 38mm carb is acceptable but the 28mm carb is not, so you will need to replace it if it is. You can figure this out by flipping the carb upside down and measuring the throat. Keihen carbs are hard to find stuff to tune them, so the SBN44 is pretty popular, except you would also need a different manifold to run the SBN carb. I would plan on spending about $100-150 to buy a SBN 44 carb and manifold (plus $70 if you need to rebuild that carb). The 750 motors with single and dual carbs also use the 38mm mikuni carbs, it wasn't until the SXR that kawasaki switched to 40mm mikuni, so don't think that going to a 750 motor will get you a different carb. For kawi, a 38 is a 38, the only thing that changes is how many. In your 4th picture is the stock keihen airbox, a keihen airfilter adapter to replace the stock airbox and a K&N filter to use with the adapter.
Your second picture is what looks like a westcoast 650 pipe. It should work with your motor. However, that manifold doesn't look like the right lord mount. Does it say westcoast on it? It will probably still bolt up at the head pipe, but not the lord mount, and the porting is probably different. The casting is very thick at the head pipe so I'm not sure that it's stock. It should be able to be opened up if it's smaller than the pipe. The waterbox looks like it may be a westcoast but I am not certain. The one hose might be right from the waterbox to the outlet but the other hos isn't right for the pipe to waterbox. Should be able to cut it down though. I would not use the silencer.
The reeds you asked about in post 5 are 10/12 bolt pattern, so they are 750 reeds.
The thing in the bottom of pic 3 is an oil pump. You have a blockoff so you need to premix, oil pumps are junk.
That is the right fuel tank. It looks like you have an aftermarket fuel pinkup in the last picture there.
Start by loosly putting the head on the motor so it stays clean, put the ski together to determine which parts you are missing, make a list but dont order any parts. Get the motor running, or at least rebuilt and ready to run with the carb you want to use, then order any missing parts, assemble everything and go ride.
