An off the shelf gear motor, speed controller, and battery pack can be used to power the cart. The sprocket to rear wheel attachment will need to be custom fabricated.
I do not see a lot of room for a sprocket to go between the edge of the tire and the rear axle hangers, so that may need to be addresses by repositioning the axle hangers inward or installing a wider axle.
A sprocket can be attached to the wheel by drilling holes through the sprocket and rim and using steel tube spacers and long bolts to attach the sprocket to the rim, or by welding steel tube spacers onto the rim and then welding the sprocket onto the steel tube spacers. Another thing to consider is installing a go kart axle and wheels onto the wagon because go kart rims already have flanges on them with holes to attach a sprocket to.
All of our throttles (with the exception of one) and handlebar switches are made for 7/8" tubing. We have one throttle made for 1" tubing. We do not have any throttles or handlebar switches made for 13/16" tubing. However 13/16" and 7/8" are very close to each other so you should be able to use thin rubber sheets as a spacer to install our 7/8" throttles and switches onto 13/16" tubing.
I ran a gear ratio calculation for this beach wagon using our item # MOT-24350G 24 Volt 350 Watt gear motor and an item # SPR-B36 36 tooth wheel sprocket and got a 4.07 MPH top speed which is considered a brisk walking speed. Here is the calculation.
Choosing the right size batteries for the kit is the next step. For a 4 hour run time two 12 Volt 50Ah or 12V 75Ah mobility scooter batteries could be used such as our item # BAT12V50A or BAT-12V75A. A 50Ah battery pack should provide a 4 hour run time on primarily flat ground with some inclines, however if you absolutely want to over-engineer the power supply so it can handle 4 hours of run time on just about any type of terrain then you may want to choose the 75Ah battery pack.
I can make a kit for this project and post a link to it here so you can see what it will look like and cost. All I need to know to make the kit is what size battery pack you have in mind?
Since the kit will use a 24 Volt motor it needs to have a battery pack made with two 12 Volt batteries. For a 1 hour run time two 12V 22Ah batteries could be used ($120 battery cost), or for a 2 hour run time two 12 Volt 35Ah batteries could be used ($160) battery cost. These run time estimates are conservative and you will most likely get more run time than I am estimating.
Since there is only a $40 difference in battery cost between a 1 and 2 hour run time I though I would let you know and give you the choice of battery packs before I estimate the cost of the kit.
Would you like an estimate for a kit with a 22Ah or 35Ah battery pack?
Please let me know if you have any questions.
The revised kit cost is $254.25. Here is a link to the revised kit: http://cart.electricscooterparts.com/ProductDetails.asp?ProductCode=KIT-147-MODIFIED
If you would like to call to order the revised kit please tell them that you would like to order item # KIT-147-MODIFIED.
Please let me know if there are any further changes that you would like made to the kit, or if you have any questions.