MiGhost is right in his identification of your bike. Check the link in my sig line to see the same bike, in pretty much stock form.
When it comes to working on these old bikes, half of the battle is having the courage to dig into a system you're not familiar with. Case in point, I bought an 84 Sportster, and was too scared to take the carb apart to make it run right, and too broke to take it to a shop. I sold it for a big loss because I couldn't bring myself to take the air cleaner off. If you've replaced a timing belt and a cooling system, there is nothing on this bike that you can't work on.
Your bike is missing some pieces. When I was in your shoes, I took it back to stock, so I knew what it's supposed to look like and do before I started modifying it.
A couple things to keep in mind as you work on this bike:
1. The CB650 was the last of the SOHC line. It was produced for only 4 years, concurrent with several DOHC models. Some consider it to be the best version of the SOHC. Each year, significant changes were made to the design. As a result, some parts can be hard to come by, and expensive when you find them. Try finding a new valve for this bike, and you'll see what I mean.
2. The CB650 falls into two major categories: 1979 and 1980 models, and 81/82 models. The major difference between the two is the carburetors. Totally different styles, as MiGhost pointed out, and completely incompatible. Your bike takes the PD50, which is a mechanical slide, similar in style to the CB750 carbs.
3. You will spend money on parts. My bike, which was more complete than yours, and I had a free parts bike on hand already, took about $1,000 to return to roadworthy status. Not trying to deter or scare you, but give a realistic picture of the kind of money you're likely to spend.
4. The money you spend on this bike, you will likely never get back out of it. Vintage bikes are a labor of love, not an investment. Since you bought the bike with the intent of learning, consider the money you put into it tuition.
5. You'll need to borrow, buy, or make some tools to do what you need to do to this bike.
6. It's OK to be excited and want to make progress. It is not OK to get so excited you start slamming things together and hoping it works. If something isn't working, or isn't coming apart or going back together well, step back, take a break, and come back later. See point 3- if you break it, you'll pay for it.
7. It's your bike. You paid for it with your money. You can do anything you want with it. You can fill the tank with lemonade and park in upside down on your lawn if you want. It's just like grocery shopping as an adult... you can fill your basket with cookies if you like, but you have to deal with the results. On the forum, you'll likely run into some strong opinions. Just look at the
steampunk/bobber thread. Remember that a forum is the online equivalent of standing up in a bar and yelling out, "Hey, anyone know anything about ______?!" There's a lot of knowledge here, but there's also some ignorance, too. In the end, it's your bike, and you can do with it as you like.
As for where to start... you're going to need a battery so you can follow James' suggestions. That's what I'd do.
