This is a VERY basic approach to making a cheep skateboard without having to know ALL of the mechanics involved. I.e. mounting the motor, the chain linkage, frame, brakes, etc; all of that is pre-made into the scooter you will use. The only additional parts I bought were: 2-9" MBS front wheels, MBS front truck, aluminum for the deck, new batteries, some cheep acrylic for the battery box, scratch tape, and misc nuts and bolts from Home Depot.
Here's a video of my buddy riding the finished skateboard down the street.
Top speed is approx: 15-21mph
Step 1: Find an Old Scooter
This is not a picture of the actual scooter I used, but it's close. I found mine on craigslist for about $50 and amazingly the only problem was the batteries were shot.
Step 2: Strip it Bare
This picture shows the remaining frame after I stripped everything off. Nothing was more difficult than unscrewing several screws, except the perpendicular steel component welded to the frame.(It supported the handle bars and LED display) I took a sawzall to it and it came off very easy. haha
Step 3: Find a Suitable Deck Material
I found four or five aluminum sheets at the local recycling scrap metal yard; they were almost the perfect size for the deck. The metal yard charged me around $20 for the material. Next, I took them to a local company that builds cotton gins and they were nice enough to offer me the service of their awesome laser cutter.
Step 4: Order New Batteries
The initial batteries were shot (3 total), so I went to Ebay and found a lot of different providers. The best price I could find at the time was around $22.
Step 5: Custom Battery Box
The initial battery box was too shallow with my new deck. So I removed the old battery box and replaced it with a custom acrylic box that I shaped into a trapezoid-ish shape.
Step 7: Pictures of separate components