I'm not sure why anyone would want to pay four times the cost of a Connex sprocket.
Connex are easily the best that I have used, and out last any other brand of sprocket.
http://www.connexchain.com/en/product/connex-e-bike-ritzel-fuer-bosch.html
If you are in the UK, you can order them via Ebay, or my preferred choice.
http://www.bike-discount.de/en/shop/e-bike-sprocket-18334
They are compatible with both Shimano and SRAM. I use on both set ups, and also have written conformation of this from the Connex technical department.
You would need to buy a locking ring, but that is no problem. I even have two locking rings with the guard sitting here doing nothing, that I removed as soon as I received my bikes. More than happy to forward one on.
The guard is just a somewhere else to clog with dirt on an eMTB, although I guess that with a 20t sprocket, riding off road in mud, or hilly terrain isn't something that you intend doing much of. The gearing will sadly be too high.
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