Speed wobbles aren't caused by a bad board. It's basically your balancing reflexes that causes it.
It starts by hitting a small bump or something. Let's say it makes you tip backwards and lose your balance. You press on your heels as a natural reflex to keep from falling over. Problem is, this makes the board turn really sharply. To compensate, you lean the other way and now you're imbalanced in the other direction. The cycle repeats until you fly off the board.
You can stop these by keeping your knees and ankles loose and consciously overcoming the balancing reflex. Turning helps too.
Got these strong-ties at a yard sale for a buck. They might become the motor mount.
Motor and battery with soda can for size reference.
With me on it: