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Modifying son's gas powered mini bike to electric

I want to convert my son's gas mini bike to electric.  It is a traditional tubular frame minibike with a 3 hp engine.  My son weighs about 60 lbs and I do not want it to go too fast.  The tires are 11 inch diameter.

I have calculated that a direct drive with a 2500 rpm motor, 11 and 80 tooth sprockets it would have a top speed of 11.22mph.

I am looking at a 36 volt 500 watt motor, controller and throttle on your website.  I think it needs three 12v 8 ah batteries.

I need some technical feedback on this set up.  Does this size motor look appropriate for this conversion?  Please make any suggestions, I am rookie at this.  Thanks!!!!!

I ran a motor power calculation and with the weight of the rider and mini bike a 500 Watt motor geared for 11.22 MPH will be able to go up a 10% incline at full speed and a 20% incline at half speed. If the mini bike needs to go up inclines steeper than 20% for an extended period of time then I recommend using a more powerful motor. 

I ran a ride time calculation which estimates that a 36 Volt 8Ah battery pack will run for 45 minutes on flat ground with a 500 Watt motor. Riding the mini bike up inclines will reduce this runtime estimate though. If you want more runtime then a battery pack made with higher Ah batteries could be used.

We have a kit which is very close to what you need with the exception of the sprockets and chain. It is our KIT-36500-01 which is available on this page: https://cart.electricscooterparts.com/ProductDetails.asp?ProductCode=KIT-36500-01

KIT-36500-01 has 9Ah batteries which are the exact same size as 8Ah batteries however they provide a 12.5% increase in ride time for only $3.00 more per battery so they are a good upgrade to make.  

Please let us know if you have any questions.

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