The best way to figure out the number of turns is to count what you get when you take the old ones off. You also need a VERY detailed diagram of how the windings were placed so you can put the new ones on in the exact same configuration. Once you get one of the windings off, you can count how many strands were used and try to measure their individual diameters. Then there are charts or formulas you can use to calculate how many strands of what gauge wire to fill the slots for a desired number of turns.
Running it at higher speed is likely to melt the plastic gear. Look for a metal one. If you gear the bike so the motor loading stays close to stock, which means not going faster than about 30mph, then the plastic gear might hold up. Above 30mph, it just takes too much power to sustain that speed regardless of motor rpm and the gears won't like it.
The stock number of turns is probably going to be good for 72v. If you put one more turn, you can get more torque for a given current, which might make the controller and battery happier.