Yamaha's lightweight, wheelie-prone TT500 is one of the all-time great off-road bikes. So how do you improve on a machine already close to perfection? When Canadian Paul Miller found himself with a basket case '77 TT500, he decided to try. He's built an elegant and nimble street tracker that, despite first impressions, is street-legal. There's hardly a square inch of this bike that hasn't been massaged: the only remaining pure TT500 parts are the engine and the front half of the frame. Miller, 25, lives in the oil town of Calgary, Alberta, and grew up around street rods and drag