AVR Freaks

Transformers work only with AC; they "sort of work" with pulsating DC (which is what you get out of typical motor speed controllers) but not reliably; they do not work with plain DC. 

So, for your application, I do not think that a transformer is the solution you need. In fact, I, personally, would not even try. Much better to use a motor that fits your battery, or a battery that fits your motor. 

With that out of the way, lets get some basics under control. 

When used for traction (eg, car, truck, scooter, bike, etc), getting the vehicle from one place to another takes a certain amount of ENERGY. It takes energy to overcome friction and to go up hills. You CAN (if your system is built to do so) get energy back going down hill. POWER is a measure of how fast ENERGY is used. For example, going up a certain hill at 10 miles per hour takes twice as much power as going up the same hill at 5 miles per hour, even though the same energy is involved in both cases (twice the speed takes half as long). 

Batteries can be characterized by the amount of energy they hold. Volts X AmpHours is one way to measure energy. Power is Volts X Amps. Theoretically, 5 batteries in series hold the same energy as when they are connected in parallel. The difference is that for a given power output, the series connection takes less current (Amps) because the voltage is higher. Parallel connection requires more current because the voltage is lower. Volts X Amps!

When you say that a motor is rated for a certain power, that is the power that it can output continuously (unless it is an intermittent power rating). In part, this is governed by its ability to get rid of waste heat. It is also governed by the size of the wires and the amount of iron in the rotating part and the fixed part. When you drive a motor with a particular voltage, it is the current that varies according to the load. More load, more current. So, if you have a motor that is rated for a certain number of kW or horsepower, it will consume less electrical power when it is lightly loaded (e.g. cruising on the flat) and more when it is more heavily loaded (going up a hill). But, in all likelihood, it will NOT consume the rated power unless it is a motor that is too small for the application. Thus, YOU have to choose a motor with the correct power rating for the application. Make it too small, and will overheat when going up a hill (or, maybe, even on the flat). Too big, and the motor will be large, heavy, and expensive (and waste more power than it should at light loads).

Motors are strange beasts. At a given voltage, a motor can do a certain amount of work and draws a corresponding current. If you INCREASE the voltage, the RPM will increase and it is possible to do more work. The current will also go up. So, two factors control the current that a motor draws: the voltage that is used to power the motor and the amount of work (load) that the motor is required to do. Typically, smaller DC industrial motors have graphs with sets of curves that show the current at different supply voltages and at different loads. Hobby motors often do not have such information. 

So, my strong recommendation is to match the battery voltage more closely to the motor, or visa versa. You will have something that works MUCH better. Also, even if you could find transformers to do what you request, at that power level, they are costly, heavy, and big.

Jim