I recently got a longboard and totally fell in love with it! Not only is it fun, but its a great workout too. The problem I was having is that sometimes I just want to go REALLY fast and not have push up all the hills in my area. My solution? Use an old broken weed eater that I had in the garage and motorize it!
Step 1: The Drive Train
My main issue that I came across pretty quickly was connecting just the wheel to the motor. I used a gear system and chain from an electric scooter and welded one gear directly to the driveshaft and another to the wheel. How I made the wheel was by driving three bolts through the wheel and using washers to spread out the force as to not destroy the wheel. From there I welded another washer to the bolts sticking out of the wheel and my drive gear onto that washer.
Step 2: Mounting
Once the drive system was working, I drilled four holes in 1/4" steel to mount onto the trucks of the board to act as a mounting plate. If you do this you will probably want longer screws because the original hardware might not do the job. I then bolted another plate to the motor to act as the adapter. You might have to drill and thread holes for it but mine just happened to bolt onto where the gas tank was (I just relocated it and zip tied it). Once you bolt both of the plates together, your motor will be solid mounted to the board.
Step 3: Final Adjustments
If your gears are tilted and the chain will not stay on, I suggest putting a bolt under AND above the plate that is attached to your motor. That way you can change the angle at which your motor is mounted.