For the current function, yes, a single transistor as a switch would work perfectly. Sorry for the quick hack-up of your diagram, but you can see what I mean below. It's a standard transistor switch circuit for switching relays, motors, whatever:
I guess you found your circuit because of the reverse function that you want - it looks like what is called an H-bridge circuit, but it is wrong. An H bridge needs pnp transistors for the top two, not npn. Your circuit, if you tried to use it as an H-bridge, would short out the battery!
Anyway, it sounds like you don't need the H-bridge, if you can manually do something for the occasions when you want to pump backwards. Here is an easy way to do that:
Simply use a DPDT (double-pole, double-throw) toggle switch connected as below:
The first shows the physical wiring, the second is a schematic showing how it works. Just connect the Vs and 0V from this in place of your motor connection in the top diagram, between transistor and power. When the toggle switch is operated, it just reverses the supply to the motor. It's a very useful little trick. Saves you messing about with the battery. Use a standard 10 amp toggle switch. You should only operate the switch while the motor is stationary though, otherwise you might get a lot of current being drawn as the motor is hard-reversed.