Here’s some instructions on how you can power a mountainboard with an electric motor.
This electric mountainboard was made with an MBS Core 95 mountainboard, an electric scooter motor, and other various parts.

There seems to have been little advancement in motorized mountainboards and off road boards over the past years. Their are many skeletons left over from motorized boards that have come and gone, and there are very few that have a solid foothold. With how things are currently in the motorized board world, you’ll [...]

If anyone knows anything about this mystery motorized mountainboard, please feel free to share. It’s pretty cool to see a motorized mountainboard that is hands free.

Nice design for a homemade mountainboard.  If you are wanting to save some money and build your own mountainboard, this might be a good design to follow.  Simple steering system, and a simple overall design.
 
Follow the link to see a well made Homemade Mountainboard.