First thing you want to do is a compression test, as others have suggested. You're going to be chasing your tail trying to diagnose anything else without knowing the state of your compression. If your compression is low, you need to find out why. Sounds like most likely rings, but could also be valves. Or both. If your compression is fine, then you've got to figure out other causes -- some of which have been stated already. Lean condition, rich condition, timing, points, valve adjust. If your plus are black and wet looking, it's oil. If they're black and sooty looking, they're rich. White smoke from the breather indicates lean (and hot). Plugs look like they're oil fouled.
You can get a decent compression gauge at an auto parts store for $30-$40 that will give you an idea of your compression. Make sure to follow instructions on how to take a reading.
Your invoice doesn't have a date on it, so don't know when the work was done. If it was relatively recently, despite the embarrassment of getting ripped off, you should go back to this guy and complain. If you can prove your bike was running better and with fewer problems than when you went in, you should get your money back. I've heard quite a few horror stories from friends in NYC who have taken vintage bikes in to shops where the mechanics have no idea what they're working with. If, on the other hand, the mechanic gave you some options ("do you want me to change out your rings?") and you said no to save money, then... lesson learned.
I'll be in NYC to work on a few bikes in mid-August. If you haven't sorted this mess out, let me know. I will be bringing a full tool kit including compression gauge and carb sync tool.
