Is there any way to allow a scooter to coast when you go down hill since the drag of the motor slows it down?

It would depend on the engine size and what type of scooter, but if it has chain drive with a clutch let the engine run at idle for maximum speed. You could put a chain idler sprocket on your wheel or get a wheel with one. If it is a gas powered scooter you can do the same, but what is slowing it down is the gearbox. If you have a goped gut a cluch kit from http://www.davesmotors.com/store/product1366.html That will fix your problem; they also have a good line of performance goped engines with clutches.

I would just turn it off if you could get it started by the end of the hill.

^while that would work, it is completely illegal.