ok i have one of these my slef but i bought mine preasembled i just looked at mine and provided you have every thing together and only the wheel needs to be installed to the motor.
try this making sure the battery is charged now reverse polarity meaning putting red wire onto negative(black) part of the battery and using the black wire connect it to the red part of the battery this will make the motor spin in the oposite direction to then hold the nut with a shifter or spanner and then turn it on and off quickly or it will strip the nut or the spanner will fly off somewhere.
when u turn it off and on it will tighten it self up but be very carful to get a good grip on the spanner or it could be dangerous only way i can think of to get it tightened up. get my drift ?? be sure to reconect the wires bak to the proper way once the nut is tightened and then its redy for use.
be very carful if you decide to do this lol could be dangerous but if you wanna try it ,it should work let me know how you went with it.