See my Delta Tripper thread for how I used a BMX rear wheel with singlespeed freewheel in a front (BMX) fork.
EDIT: added:
Assuming the BMX wheel would fit in your existing fork, along with the freewheel for it,, but the wheel is too small for you, then you could simply build a new wheel out of that hub and your preferred rim.
If the hub fits but not the freewheel, then usea thread-on sprocket without freewheel, and put the freewheel on the motor end of things.