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Simon,

I would consider picking up a Graupner 700 8.4v BB motor and a flex cable, .150 with 3/16 prop shaft over a straight shaft. Then use a 3/16" strut. The prop center line should be close to the bottom of the rear ride pads or even slightly below/above. You may angle the prop slight downwards. Start with an Octura X440. Water cool the can and brushes on the motor.

On 12 cells, you should be around 25mph.

Ryan