Unfortunately, you are pretty much stuck with the tiny front sprocket. A larger one will rub the frame at the chainstay. It's one of the reasons I sold off my EVG, instead of just replacing the motor with something stronger and faster. Otherwise a great frame! Wonderfully strong, and all the electonic parts except the throttle are nice and dry inside the covers.
My EVG came with no controller present, so I ran it on a generic brushless controller that ran at 24-48v. The best way to guess whay your stock controllers max may be is to look at the capacitors inside it, if the case is openeable. The caps look like tiny cans, and have a voltage marking. If it's sealed, my guess is that it has at least 30v caps inside, So an 8 cell (8s) lifepo4 battery would work fine with it. Might look at this site, for a really good one, http://www.emissions-free.com/ Or another good option would be a 15 ah lifepo4 from pingbattery.com Ditch the lead unless you are really on a tight budget, or need only a few miles range.
The EVG's 24v heinsmann motor does a great job of doing what it is designed for. 12 mph travel, but with excellent hill climbing ability. But it's permanently geared for that, so if you upvolt it to a faster 36v, you end up with a bike you don't pedal. You just sit and ride.
One very viable option though, is to basicly scrap the whole bike, ( which can eventually be sold as parts on ebay for decent money) and then take just the motor and build a faster bike from scratch. New 36v 15 amp controller, new 36v 20 ah lifepo4 battery, and an MTB or comfort bike frame. Then you get 20 mph or so, and gearing to pedal that fast.
A lot depends on what you actually need in an ebike. As is, the bike makes a great tool for rides in the 6 mile roundtrip ballpark. Less speed is needed for a short range bike. you can replace the battery easy. 12v 12 ah batteries for kids toy electric cars fit perfect. Get em at wallmart, or from an on line battery retailer.