How could I power my custom E-bicycle project?

[A RC Dude]

hey guys, i'm working on a custom electric bicycle (obviously) but am having trouble finding a good way to provide power to the motor. it is a 2.5hp (1865W), 12V ramsey winch motor. I have both a car battery and my go-cart battery i can use to power it without having to spend any extra money (very tight budget). i've got all the mechanical stuff figured out (not built yet though) but i need a way of controling the power going to the motor. It would be ideal to have a way to adjust the speed going to the motor but i can just have some kind of push botton hooked to a solinoid for a simple on/off function. So, my questions:

The manufacture doesn't give any efficiency or amp specs on the motor but if watts=volts*amps*efficiency and we guess that the motor is 60% efficient, then it should pull around 259amps, right?

1. is their any kind of potentiometer that could withstand that many amps?

2. Is their any other way to limit the power to the motor in steps?

3. Are their any solinoids that can handle that many amps continuously? I've found several 150A continuous duty solinoids but non any heigher.

4. Can i split all the wires between two identical solinoids so that each one only has to handle half the current?

5. Any other ideas? lol

any imput would be very helpful! I know some basic electrical theory but i'm not by any means an exspert. I don't want to have a solinoid catch fire on me while riding my bike across campus or anything lol thanks for any imput :)