The below kit might be of interest. You probably could bypass the pot and timing chip and drive the h-bridge directly from the arduino.
Yeah - you probably could; though how is a big question, since I don't see any manual or assembly guide for the kit to download from the site. Hopefully you get a guide with the kit is detailed enough to figure out such an interface, otherwise you would have to reverse-engineer the schematic. I am not sure the OP, though, would be up to either.
Something that should be noted about the specs given (bolding mine):
SPECIFICATIONS
Voltage: The kit and motor use the same power supply.
Since the maximum operating voltage of the LM324 is 32VDC then this is also the maximum voltage available to run the motor.
Current: The IRFZ44 MOSFET can handle 49A; the IRF4905 can handle 74A.
However the PCB tracks that run from the MOSFET pins to the screw terminal block can only handle around 5A.
If you do then check that the MOSFETs don't get too hot; if so then bigger heatsinks will be required.
Note that while the MOSFETs can handle the current, the tracks on the PCB can't (for the OP's motors, anyhow) - so you would have to do something differently with the circuit to get it to work with the larger motors (either a redesign of the PCB, or moving the MOSFETs off-PCB, perhaps with a larger heatsink, etc). With the OP's motors, you would probably smoke the PCB traces on the first power-up from dead start (essentially a stall condition).
