Electric drivetrain , clutch question.


I am putting together an electric drive system on an old 4 wheel surrey bike. My current plan is to use a 750 watt 580 RPM gear head motor and controller. I have the following plan, but may have discovered an issue. I will try to explain the set up, and then my issue.  Motor has a gear head and a 10 tooth sprocket and I have an axle with three sprockets.  I am running the motor to an axle with a 17 tooth sprocket in the center with two 21 tooth sprockets on each end. The 21 tooth sprockets go to the 26 tooth sprockets on the rear wheels. rear wheels are 18' and this should give me about 12-14 RPM top end.This is a freewheeling sprocket on each wheel. My issue is the motor does not spin freely when not under power, and so when motor off, the kart won't be able to be moved very easy. Is there some sort of clutch I can add??  Maybe I need to go to a high RPM Motor, and change my gearing.  See my simple drawing

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