Hi,
Might be better if you say what the actual fault is, eg the motor does simply not run or control is erratic or the control panel does not respond etc?
Depending on the fault, it might be worth inspecting all the wires/connectors and pcbs for dry joints as the constant vibration it has to cope with can cause such failures.
If all else fails, as Max says it might be possible to run the Motor Control Board from a simple pot, bypassing the main control panel, but again without knowing the actual fault its hard to say.
Often its simpler and cheaper to buy a working second hand treadmill, you can get some good bargains if you search around.