Hi Fastpace,
You need to build your own H bridge.
you could use the Arduino to control an H bridge IC driver like this:
http://www.intersil.com/data/an/an9405.pdf
And use mosfets like these http://www.irf.com/product-info/datasheets/data/irf3205.pdf
Also note that you need proper high voltage protection diodes to protect the mosfets and driver ic's from high voltage flyback/inductive collapse spikes.
Info Part 1 | Modular Circuits
You should probability plan for much more than 160watt.
Dc motors can have very high amp draw while startup.
And when you stall them the only current limiting factor is the motor coil resistance which could very well be below a few ohms. So if that motor has a 1 ohm coil and you supply 24 at 100% throttle the watt could reach 576watt.
Regards,
Steven